White House national security spokesman John Kirby on Tuesday said that the United States has not seen any indication that Russian President Vladimir Putin has made a decision on whether or not to use nuclear weapons amid the war in Ukraine, but that recent comments make it “imperative” for the U.S. to monitor the situation.

“We’ve seen no indication, nothing that tells us Mr. Putin has made a decision to use nuclear weapons or weapons of mass destruction at all inside Ukraine,” he told reporters. “And, we’ve seen no indication that the Russian apparatus is moving anything to prepare for that kind of a decision.”

Kirby said that his information is based on the U.S. monitoring Putin’s capabilities, which it has done since the beginning of the conflict. Additionally, he said that the U.S. has not seen anything that would cause it to change its strategic deterrence posture to defend itself or allies and partners.

Questions over Putin using nuclear weapons have escalated since the Russian president said that Moscow was prepared to use nuclear weapons to defend any of its territory, accusing the U.S. and its allies last month of “nuclear blackmail.”

“Obviously, we take these threats seriously,” Kirby said on Tuesday. “He’s now commented more than once, in just the last week or so, and we know that he knows his military is under increasing pressure inside Ukraine. So all of that, we believe, makes it imperative that we continue to monitor those capabilities as best we can. And we’re doing that.”

Kirby also reiterated that the U.S. does not seek conflict with Russia, will not send American troops to fight inside Ukraine, and that “no one wants to see this war escalate into the nuclear realm.”

President Biden last week warned that Putin was “not joking” in his references to using nuclear weapons, saying Russia would use such weapons because his military “is, you might say, significantly underperforming.”

Biden and allies vowed on Tuesday, in a virtual meeting with Ukrainian President Volodymyr Zelensky, to hold Russia accountable if it follows throughout on threats from the country of using nuclear weapons amid the war in Ukraine. 

The meeting comes as Putin has launched dozens of missiles that landed in cities across Ukraine on Monday, including Kyiv, and continuing into Tuesday.

Zelensky said on Saturday that Russia has begun to prepare its people for the possibility of the use of nuclear weapons against Ukraine and the Ukrainian president denied claims that he had called for preemptive nuclear strikes against Russia, saying that what he said was mistranslated.