Russia Claims it Destroyed Ukrainian Air Defense SystemsRussia said Monday that it destroyed several air defense systems in Ukraine over the weekend, ahead of what Ukrainian President Volodymyr Zelenskyy is warning could be the start of a renewed Russian offensive into the country's eastern region. The Russian claims could not be verified, but Russian Defense Ministry spokesman Major General Igor Konashenkov said the military launched cruise missiles to destroy four launchers Sunday on the southern outskirts of the central city of Dnipro and also hit systems in the Mykolaiv and Kharkiv regions. Zelenskyy pleaded in a video address Monday for more military aid from the U.S. and its allies, specifically asking South Korean lawmakers to send more equipment that can shoot down Russian missiles. He also said tens of thousands of people have been killed in Russia's assault on the southeastern port city of Mariupol. Pentagon spokesperson John Kirby said the United States was unable to confirm Ukrainian casualty figures in Mariupol because fighting there is ongoing but said the final death toll could be "a significant number." The Pentagon said Monday it is seeing early signs of efforts by Russia to reinforce its troops in eastern Ukraine. The U.S. spotted a convoy north of Izyum with command-and-control elements, enablers, artillery and rotary blade air support, according to a senior U.S. defense official. The official said there also are indications that Russian forces sent from northern Ukraine to Belarus and the Russian town of Valuyki are now moving toward eastern Ukraine. The official said, "We do not assess a new offensive has started" in eastern Ukraine but added, "What is clear is that the Russians continue to sink to new lows of depravity and brutality as we saw with the missile strike on a train station last week and their continued assault on Mariupol. "We're certainly bracing ourselves here for some potentially really, really horrible outcomes," the official said. The U.S. official said Russia has launched 1,500 missile attacks on Ukraine during 47 days of war, destroying apartment buildings and hospitals throughout the country and killing thousands of Ukrainian civilians. Russia has acknowledged sustaining "significant" troop losses of its own. Russia said one of its latest missile attacks hit four S-300 launchers provided by a European country it didn't name. Slovakia gave Ukraine just such a system last week but denied it had been destroyed. Military analysts say that Russia's failure to capture the Ukrainian capital of Kyiv and the bulk of territory throughout the country has in part been the result of its failure to control the skies over Ukraine to provide cover for ground troops. Nonetheless, its missile bombardment has virtually flattened some cities, and one missile attack on a train station killed more than 50 people last Friday. Russia has not been able to stop the flow of more military aid to Ukraine, with eight to 10 flights arriving daily. Pentagon spokesperson Kirby said that U.S. security assistance is getting to Ukraine in as little as four to six days. "That's an incredible rate of speed, and we aren't slowing down," he said. Zelenskyy told the CBS News show "60 Minutes" on Sunday that Ukraine's fate depends on further Western military assistance. "To be honest, whether we will be able to (survive) depends on this," Zelenskyy said. "Unfortunately, I don't have the confidence that we will be receiving everything we need." European Union foreign ministers are meeting Monday to discuss another round of sanctions against Russia. Meanwhile, Austrian Chancellor Karl Nehammer met with Russian President Vladimir Putin in Moscow, the first such visit by a European leader since Putin launched the invasion of Ukraine on February 24. In a statement after the meeting, Nehammer said the discussion with Putin was "very direct, open and tough." Nehammer said his most important message to the Russian leader was that the war in Ukraine must end because "in a war there are only losers on both sides." There was no immediate comment from the Kremlin. Some information in this report came from Reuters and The Associated Press.