US Spy Chief Warns of Russia's Nuclear Threats, 'Potential Desperation' WASHINGTON — Russia’s nuclear saber-rattling has the attention of top U.S. intelligence officials, some of whom are expressing growing concern about the Kremlin’s willingness to unleash some of its nuclear arsenal as it faces “potential desperation” in Ukraine. Central Intelligence Agency Director William Burns warned Thursday that the world should not underestimate Russian President Vladimir Putin, whose appetite for risk has only grown “as his grip on Russia has tightened.” "Given the potential desperation of President Putin and the Russian leadership given the setbacks that they've faced so far militarily, none of us can take lightly the threat posed by a potential resort to tactical nuclear weapons or low-yield nuclear weapons," Burns said during a speech to students at Georgia Institute of Technology in Atlanta, also known as Georgia Tech. "We're obviously very concerned," he said, noting that Putin has an “almost mystical belief that his destiny is to restore Russia's sphere of influence," which includes bringing Ukraine under the Kremlin’s sway. Russia first put its nuclear deterrence forces on high alert on February 27, just three days after sending troops into Ukraine, citing aggressive statements by NATO and economic sanctions from the West. Another warning On Thursday, one of Putin’s closest allies further warned that Russia would place nuclear warheads in the Baltics should Sweden and Finland decide to join the Atlantic alliance. "There can be no more talk of any nuclear-free status for the Baltic — the balance must be restored," said Dmitry Medvedev, deputy chairman of Russia's Security Council. Medvedev said Russia would place warheads in Kaliningrad, a Russian exclave on the Baltic Sea that is 500 kilometers from Berlin and less than 1,400 kilometers from London and Paris. Lithuania’s defense minister downplayed the Russian threat, calling it nothing new. "Nuclear weapons have always been kept in Kaliningrad ... the international community, the countries in the region, are perfectly aware of this,” Arvydas Anusauskas told Lithuania's BNS news service. 'Troubled' Still, the U.S. spy chief Thursday warned that Russia’s nuclear threats bear watching. "I have learned over the years never to underestimate Putin's relentless determination, especially on Ukraine," said Burns, who met with Putin in Moscow last November, hoping to dissuade the Russian leader from invading Ukraine. "I was troubled by what I heard," Burns said. Despite his concern, the CIA director said U.S. intelligence has yet to see evidence Moscow is preparing to unleash any part of its nuclear arsenal. "While we've seen some rhetorical posturing on the part of the Kremlin, moving to higher nuclear alert levels, so far we haven't seen a lot of practical evidence of the kind of deployments or military dispositions that would reinforce that concern," he said. U.S. defense officials have previously described Russia’s decision to put its nuclear deterrence forces on high alert in February as "escalatory and unnecessary," but say they have yet to see anything that would require a U.S. response. "We're obviously watching that very closely," Pentagon press secretary John Kirby told reporters during a briefing Thursday. "We have seen nothing in the space out there that has given us cause to change that [nuclear deterrence] posture in any tangible way." "It is not something that we take for granted," he added. The latest U.S. security package for Ukraine, approved Wednesday, includes gear designed to protect Ukrainian forces from nuclear, biological and chemical exposure. Russian war crimes During his speech Thursday, Burns warned "the last chapter in Putin’s war has yet to be written," saying that Russia was unlikely to depart from its current, vicious strategies. "I have no doubt about the cruel pain and damage that Putin can continue to inflict on Ukraine, or the raw brutality with which Russian force is being applied," he said. China The U.S. spy chief also called out China as "a silent partner in Putin's aggression" in Ukraine, adding that Beijing presents a threat to the United States and the West in its own right. China "is our greatest challenge, in many ways the most profound test the CIA has ever faced," Burns said. "As an intelligence service, we have never had to deal with an adversary with more reach in more domains." Some information for this report came from Reuters.