Story highlights U.S. official: G7 sanctions will be "coordinated," "not necessarily identical" 
 
 Russian fighter jets enter Ukrainian airspace on several occasions 
 
 Ukraine official announces "second stage" of operation against militants in Slavyansk 
 
 President Barack Obama says targeted sanctions against Russia are "ready to go" 
 
 Leaders of the G7 nations on Friday condemned Russian actions in and around Ukraine, promising punishments aimed at sending a message to Moscow. 
 
 "We have now agreed that we will move swiftly to impose additional sanctions on Russia," the G7 leaders said. 
 
 "Given the urgency of securing the opportunity for a successful and peaceful democratic vote next month in Ukraine's presidential elections, we have committed to act urgently to intensify targeted sanctions and measures to increase the costs of Russia's actions." 
 
 The statement from the group -- which includes Canada, France, Germany, Italy, Japan, the United Kingdom and the United States -- came hours after U.S. President Barack Obama threatened Russia on Friday with new sanctions. 
 
 According to a senior Obama administration official, each G7 country "will determine which targeted sanctions they will impose" -- measures that "will be coordinated and complementary, but not necessarily identical." 
 
 The United States could take action as early as Monday, according to the official. 
 
 This comes as Western nations and Ukraine's government in Kiev express growing concerns about what they see as Russia's role in the Eastern European nation, its military movements outside it and the potential for the situation becoming an even larger conflict that could envelop the region. 
 
 Ukrainian Prime Minister Arseniy Yatsenyuk accused Russia of aiming to occupy his country "militarily and politically," claiming that Russia wants to start a third world war. 
 
 His strong statement came amid a sharp escalation in tensions. 
 
 JUST WATCHED US: Russian plane enters Ukraine Replay More Videos ... MUST WATCH US: Russian plane enters Ukraine 00:34 
 
 JUST WATCHED Tense moments at Ukrainian checkpoint Replay More Videos ... MUST WATCH Tense moments at Ukrainian checkpoint 02:05 
 
 JUST WATCHED Russia orders new military drills Replay More Videos ... MUST WATCH Russia orders new military drills 01:34 
 
 JUST WATCHED Kerry: Russia not keeping its word Replay More Videos ... MUST WATCH Kerry: Russia not keeping its word 01:02 
 
 Russia's defense chief ordered new military drills Thursday near the border with eastern Ukraine after Ukrainian forces said they killed five pro-Russian militants in an operation to clear roadblocks near the city of Slavyansk. 
 
 Serhiy Pashinskiy, chief of staff to acting Ukrainian President Oleksandr Turchynov, said Friday the drills had entered their "second stage," aimed at encircling Slavyansk and cutting off additional supplies or support. 
 
 And on Friday afternoon, Pentagon spokesman Col. Steve Warren said Russian aircraft entered Ukrainian airspace on several occasions over the previous 24 hours. He urged Russia "to take immediate steps to de-escalate the situation." 
 
 A U.S. official said they were fighter jets. They were not major incursions, but rather a handful of flights along the border that crossed into Ukrainian airspace, the official said. 
 
 Yatsenyuk said he had asked Russia to explain the presence of its troops on Ukraine's border within 48 hours -- but 24 hours later, Kiev was still waiting for an official response. 
 
 At the same time, he said, the world is lined up with Kiev in its efforts to restore stability and preserve "the system of world security that Russia wants to destroy." 
 
 Just as Kiev announced the second phase of its so called "anti-terrorism operation," behind the barricades of the security services building, at the command and control center, the pro-Russian forces expressed confidence they can repel any sort of attack. 
 
 "We have 2,000 fighters at the ready and plenty of weapons," says Evgeniy Gorbik, a former military man turned spokesman for pro-Russian militants. 
 
 On Friday, Ukraine's Interior Ministry reported that 13 people on board a bus -- including foreign observers and military officials -- were detained at the entrance to the town of Slavyansk. The ministry said the captors were unknown, but that the detainees were taken to a security office that is under the control of pro-Russian separatists. 
 
 The detainees are seven European representatives from the Organization for Security and Co-operation , five Ukrainian military representatives and a bus driver, the ministry said. 
 
 "We strongly condemn the fact that the members of the Organization for Security and Co-operation mission were taken hostage by pro-Russian separatist forces, this is completely unacceptable," Ukrainian Deputy Foreign Minister Danylo Lubkivsky told CNN. 
 
 Negotiations were ongoing to secure their release, the ministry reported. 
 
 U.S. pressure 
 
 Meanwhile, top U.S. officials have vowed more punitive international actions targeting Russia's economy if Moscow, in their view, continues to escalate the situation. 
 
 The war of words between the United States and Russia is growing increasingly intense. 
 
 In a phone call Friday, Obama and European leaders agreed that Russia has failed to do its part to de-escalate tensions, the White House said. 
 
 The Russians have not supported a deal reached last week to decrease tensions, nor have they called on militant groups to put down their weapons, the White House said. To the contrary, Russia has made things worse through its rhetoric and military exercises, it said. 
 
 At a news conference Friday in South Korea, Obama said there will be targeted sanctions that are "ready to go." 
 
 "I think it's important for us not to anticipate that the targeted sanctions that we're applying now necessarily solve the problem," he said. 
 
 "What we've been trying to do is continually raise the costs for Russia of their actions while still leaving the possibility of them moving in a different direction. And we will continue to keep some arrows in our quiver in the event that we see a further deterioration of the situation over the next several days or weeks." 
 
 Obama also praised the unity of other countries in condemning Russian "meddling" in Ukraine. 
 
 Moscow argues that the government in Kiev, which took power after ousted pro-Moscow President Viktor Yanukovych fled Ukraine in February, is illegitimate. 
 
 The United States has accused Russia of supporting, arming and coordinating the separatist militants -- an allegation Moscow denies, saying these are simply local "self-defense" units. NATO and the United States have also voiced concern over an estimated 40,000 Russian troops massed near the Ukrainian border. 
 
 Russia in turn accuses the United States of "running the show" in Ukraine and says it must do more to hold Kiev to its side of a deal reached in Geneva, Switzerland, saying the interim government must disband right-wing ultranationalist groups. 
 
 On Friday, the International Criminal Court said chief prosecutor Fatou Bensouda had opened a preliminary investigation into alleged crimes committed in Ukraine while Yanukovych was in power. 
 
 The investigation will look at the period from November 21 to February 22, when street protests against Yanukovych's government took place that resulted in bloody clashes with security forces. 
 
 Volatile situation 
 
 Only a week has passed since foreign ministers for Ukraine, Russia, the United States and the European Union agreed to the Geneva deal aimed at easing the crisis. 
 
 But that pact, which called for all sides to refrain from violence and for illegal militia groups to disarm and leave occupied buildings, appears to have faltered, if not failed. 
 
 Pro-Russian militants remain holed up in government buildings in around a dozen cities in eastern Ukraine. 
 
 Slavyansk has been at the center of much of the tension. 
 
 Nearby in Kramatorsk, Russia's state-run RIA Novosti news agency reported gunfire at an airfield that Ukrainian forces retook from separatist protesters this month. 
 
 But the head of Ukraine's anti-terror operations unit, Vasil Krutow, told CNN that a helicopter at the airfield had exploded after a shot hit its fuel tank. He said the crew's commander was injured as a result. 
 
 An explosion was also reported overnight at a police checkpoint in the southwestern city of Odessa. 
 
 Economic impact 
 
 Unless and until additional U.S. sanctions are announced, it's hard to judge how much they will hurt Russia. Previous rounds targeted individual lawmakers and businessmen seen as close to Putin as well as a bank, Rossiya, believed to serve the Russian President and senior officials. 
 
 The European Union has also imposed its own asset freezes and visa bans. 
 
 Russia increased interest rates for a second month running Friday in an effort to limit the economic damage of rising tensions over Ukraine. The move came hours after ratings agency Standard & Poor's cut Russia's credit rating to one notch above junk, citing a flight of capital from the country. 
 
 The Russian ruble lost more ground Friday, taking its losses against the dollar to 8.4% since the start of the year. 
 
 "We do believe that by more economic pressure, we can stop the aggressor," said Danylo Lubkivsky, Ukrainian Deputy Foreign Minister. 
 
 Any further sanctions could hurt the European Union as well as Russia, since the two have deep trade ties and nearly a third of Europe's natural gas comes from Moscow. 
 
 Russian Prime Minister Dmitry Medvedev acknowledged this week that the sanctions, coupled with other factors, were affecting the economy. ||||| A Ukrainian police officer stands guard at a checkpoint, with Ukrainian flags, that was attacked by unknown men outside the Black Sea port of Odessa, Ukraine, on April 25, 2014. Seven people were injured in the attack. (Photo: Sergei Poliakov AP) 
 
 The United States and other nations in the Group of Seven say they have agreed to "move swiftly" to impose additional economic sanctions on Russia in response to its actions in Ukraine. 
 
 In a joint statement released Friday night by the White House, the G-7 nations say they will act urgently to intensify "targeted sanctions." The statement says that the G-7 will also continue to prepare broader sanctions on key Russian economic sectors if Moscow takes more aggressive action. 
 
 The White House says U.S. sanctions could be levied as early as Monday. 
 
 The G-7 nations say they are moving forward on the targeted sanctions now because of the urgency of securing plans for Ukraine to hold presidential elections next month. 
 
 Russian warplanes have entered Ukrainian airspace several times in the last 24 hours, the Pentagon announced Friday. 
 
 U.S. officials called on Russia to cease the operations, said Army Col. Steve Warren, a Pentagon spokesman. 
 
 "We call upon the Russians to take immediate steps to de-escalate the situation," Warren said. 
 
 Army Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, spoke with his Russian counterpart on Thursday, but details of their conversation were not released. Defense Secretary Chuck Hagel has tried, without success, to contact Russia's defense minister. 
 
 Ukraine's air defenses are old but still potent enough, under the right circumstances, to take down Russian aircraft, said Michael O'Hanlon, a military analyst at the Brookings Institution. In fact, drawing fire might be Russia's intent by entering Ukraine's airspace. 
 
 "I don't put a deliberate provocation, in order to create a pretext for response and invasion, past Putin at this point," O'Hanlon said. 
 
 The U.S. response to such a move, O'Hanlon said, would be limited, perhaps the sale of arms or sharing intelligence. 
 
 The violation of Ukraine's airspace follows war games that Russian troops began near the border. About 40,000 Russian troops have massed there, according to the Pentagon. Last month, Russian troops seized key facilities on the Crimean peninsula and annexed it. 
 
 Tensions in the region have escalated steadily since then. Russian-backed separatists have seized government facilities in eastern Ukraine, prompting Ukrainian forces to try to evict them. Several people have been killed in those clashes. 
 
 Earlier this week, the Pentagon deployed 600 paratroopers to Poland and three other Baltic states to reassure NATO allies in the region about the U.S. commitment to their defense. 
 
 Ukrainian Prime Minister Arseniy Yatsenyuk charged Friday that Moscow "wants to start World War III" by seeking to take over Ukraine militarily and politically. 
 
 "Russia behaves as an armed gangster, but the time will come when the gangster will be hit in the face," he said at a government meeting Friday. "The aggressive efforts of the Russian military on Ukraine's soil will lead to a conflict on European soil." 
 
 "The world has not yet forgotten World War II, but Russia already wants to start World War III," Yatsenyuk said. 
 
 The prime minister accused Russia of organizing the unrest in Ukraine's east to sabotage Ukrainian presidential elections, scheduled for May 25 when the country will vote on a replacement for former president Viktor Yanukovych, who was ousted following street protests in February. 
 
 Over the past two weeks armed pro-Russia protesters have taken over some key administrative buildings in Ukraine's eastern Donetsk region, demanding a referendum on the region joining Russia. 
 
 Ukrainian officials have taken back some buildings in ongoing anti-terrorist operations that resumed Tuesday after separatists were accused of murdering a pro-Kiev local official, whose body was found near the separatist-controlled city of Slovyansk. 
 
 In the latest incident on Friday, a Ukrainian military helicopter exploded at an airfield in Ukraine's east after it was shot at by a grenade-launcher, said the country's Defense Ministry. 
 
 Pro-Russian protesters had stormed the same airfield April 15. At the time, the Ukrainian army pushed them back, wounding three protesters, to the dismay of some 300 locals who blamed the army for opening fire. 
 
 The Defense Ministry is investigating the incident, but has not released information on who they think is behind the attack. Nobody was killed in the incident, which came after Ukrainian law enforcers killed five separatists, while attacking their roadblock in the east. 
 
 CAPTIVE: U.S. journalist beaten, blindfolded by abductors 
 
 CENTRAL BANK: Russia raises interest rate to 7.5% 
 
 In another development, a Ukrainian foreign ministry spokesman, Yevhen Perebyinis, said at a briefing on Friday that contact had been lost with members of a military verification mission of the Organization for Security and Cooperation in Europe (OSCE) in Donetsk region, in the east. 
 
 Perebyinis said that "according to preliminary report, they could be captured by terrorists," Interfax Ukraine reports. The OSCE mission was sent into disputed areas to try to calm tensions. There have been no confirmation of the report that the members of the team may have run into trouble.forts of the Russian military on Ukraine's soil will lead to a conflict on European soil." 
 
 Meanwhile, Russia announced new military exercises Thursday involving ground and air forces near its border with Ukraine. 
 
 Russian Foreign Minister Sergey Lavrov accused the West of plotting to control Ukraine to bolster its own interests in the region. 
 
 "The West wants -- and this is how it all began -- to seize control of Ukraine because of their own political ambitions, not in the interests of the Ukrainian people," Lavrov said. 
 
 As tensions mounted, seven people were injured early Friday when a hand grenade was tossed at a checkpoint manned by pro-Ukrainian activists outside the Black Sea port of Odessa in southeastern Ukraine. 
 
 Local police spokesman Volodymyr Shablienko said unknown men threw the explosive device at a checkpoint set up by local authorities and activists. 
 
 Odessa residents have built several checkpoints outside of the city in an effort to block pro-Russian separatists entering from Moldova's breakaway territory of Transdniestria. The enclave, which declared indeendence in the early 1990s, is located about 50 miles west of Odessa. It is home to Russian peacekeepers and Russian troops guarding a cache of Soviet-era arms. 
 
 A senior official traveling in Asia with President Barack Obama said he is likely to call European leaders Friday to discuss the possibility of further economic sanctions on Russia. The official spoke on condition of anonymity because there had been no official announcement. 
 
 Secretary of State John Kerry charged on Thursday that Russia is not abiding by last week's Geneva agreement to de-escalate the crisis in Ukraine. Russia, he said, "has refused to take a single step in the right direction." 
 
 "If Russia continues in this direction, it will be not just a grave mistake, it will be an expensive mistake," he told reporters at the State Department. "The world will make sure costs for Russia will only grow." 
 
 In another development, Ukraine's State Security Service announced Friday that it had arrested two Ukrainians allegedly working for Russian intelligence services. One of the arrested was an officer of the Interior Ministry's Internal Troops, who was assigned with getting information on internal troop locations. 
 
 The internal troops are under the control of the Interior Ministry and are the main force behind the counter-terrorist operation lead by Ukraine's government in the east against armed separatists. The operation was suspended for a few days after Ukraine and Russia agreed on measures to deescalate tensions in the region in Geneva April 17. 
 
 Russia has hinted it may send troops into the region to protect ethnic Russians, but a new poll published by the Washington-based International Republican Institute showed that 69% of Ukrainians living in the country's troubled east are against Russian troops entering Ukraine. 
 
 Some in the east are also frustrated with a perceived lack of progress against the pro-Moscow protests. 
 
 "I say -- if we have a state, it must restore order, and not leave us one on one with these people," said Natalia Afanasieva, who lives in Horlivka, a city the Donetsk region." 
 
 Contributing: Associated Press 
 
 Read or Share this story: http://usat.ly/1gWZxob
– Russia's incursion into the Ukraine could easily spill past its borders, Prime Minister Arseniy Yatsenyuk said today, amping up his rhetoric against the Kremlin. "Attempts at military conflict in Ukraine will lead to a military conflict in Europe," he warned, in a TV appearance and web statement, according to USA Today. He said Russia planned to initiate just such a conflict, after first undermining Ukraine's elections and destabilizing it politically. "The world hasn’t forgotten the World War II, but Russia already wants to start World War III." Meanwhile: President Oleksander Turchynov's chief of staff said that the military operation on Slovyansk had entered its "second stage," with troops encircling the city and cutting off all supplies, CNN reports. He also said that Russia had refused to explain the presence of troops on the border. A bus with 13 people on board—including foreign observers—was stopped outside Slovyansk, and its passengers detained. It's unclear who did the detaining. New York Times reporters in the city say Ukrainian infantry units are patrolling nearby farmland, and well-armed Ukrainian forces are positioned on the road to Izyum north of Slovyansk. Barack Obama backed up John Kerry's sanctions threat in a news conference in South Korea today. "We will continue to keep some arrows in our quiver," he said, promising that more punitive measures were "ready to go."