The White House warned Russia that using tactical or strategic nuclear weapons in Ukraine would have “catastrophic consequences,” according to a top Biden administration official, after Russian President Vladimir Putin threatened to use nuclear force.

“We have communicated directly, privately, and at very high levels to the Kremlin that any use of nuclear weapons will result in catastrophic consequences for Russia,” White House national security adviser Jake Sullivan told CBS’ “Face The Nation” on Sunday.

“The United States and our allies will respond decisively,” Sullivan said, “and we have been clear and specific about what that will entail.”

“We have been equally clear in public, as a matter of principle, that if Russia uses nuclear weapons, the United States will respond decisively, and that we will continue to support Ukraine in its efforts to defend its country and defend its democracy.”

He went on to say that, while Putin has played the “nuclear card” at various points throughout the seven-month-long war, the Biden administration is taking Putin’s latest threats “deadly seriously.”

Russia has the world’s largest nuclear arsenal, which includes strategic nuclear weapons for use against targets such as bases and cities, as well as tactical nuclear weapons for use on the battlefield. Russia, with an operational nuclear triad, can launch nuclear strikes from land, air, and sea.

If Russia uses nuclear weapons, it will be the first time since World War II, when the United States dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki.

Sullivan’s remarks follow Putin’s rare televised address last week, in which he announced a partial military mobilization of the country’s reservists and threatened to use nuclear weapons. The Russian leader falsely accused Western countries of “nuclear blackmail,” claiming that they are deliberately trying to provoke the Kremlin with heightened rhetoric about their own nuclear weapons stockpiles.

“To those who make such statements about Russia, I would like to remind you that our country has various means of destruction, some of which are more modern than those of NATO countries,” Putin said.

“And if our country’s territorial integrity is threatened, we will certainly use all available means to protect Russia and our people,” he said, adding that “this is not a bluff.”

Since Russia’s invasion of Ukraine in late February, Western officials have repeatedly warned Putin that if he feels backed into a corner, he may resort to using nuclear weapons.

“Given President Putin and the Russian leadership’s potential desperation, 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,” CIA Director William Burns said in April.

Similar concerns have been expressed by some Russia experts, particularly as the Russian military struggles in the face of a Ukrainian counteroffensive. Former defense attaché to Russia retired Brig. Gen. Kevin Ryan told Insider in mid-September that Putin’s use of nuclear weapons has made him “even more concerned.”

Others who follow Russia are skeptical that Putin would use weapons of mass destruction.

“Anyone who finds it necessary to say that he’s not bluffing most likely is,” Ivo Daalder, a former US ambassador to NATO, told Insider last Wednesday.

Retired Adm. James Stavridis, a former NATO supreme allied commander, told MSNBC on Friday that Putin’s nuclear threats do not cause him to lose “a lot of sleep.” “I don’t see Putin using nuclear weapons,” he said.