The US told Russia it will face "catastrophic consequences" if it uses nuclear weapons in Ukraine.
A Biden administration official said the White House recently warned the Kremlin of such a response.
Putin threatened last week to use nukes as he announced a partial military mobilization and other measures.
The White House told Russia that it will face "catastrophic consequences" if it moves to use tactical or strategic nuclear weapons in Ukraine, a top Biden administration official said, after Russian President Vladimir Putin warned that the use of nuclear force was on the table.
"We have communicated directly, privately and at very high levels to the Kremlin that any use of nuclear weapons will be met with catastrophic consequences for Russia," Jake Sullivan, the White House national security adviser, told CBS' 'Face The Nation' on Sunday.
"The US and our allies will respond decisively, and we have been clear and specific about what that will entail," Sullivan said. "We have, in public, been equally clear, as a matter of principle, that the United States will respond decisively if Russia uses nuclear weapons and that we will continue to support Ukraine in its efforts to defend its country and defend its democracy."
He added that although Putin has been waving the "nuclear card" at various points throughout the devastating seven-month-long war, the Biden administration is taking Putin's latest threats "deadly seriously."
Russia maintains the world's largest nuclear arsenal, which consists of strategic nuclear weapons for use against targets like bases and cities and tactical nuclear weapons for battlefield use. With an operational nuclear triad, Russia has the ability to conduct nuclear strikes from land, air, and sea.
Were Russia to use nuclear force, it would mark the first time nuclear weapons have been used since World War II, when the US dropped atomic bombs on the Japanese cities Hiroshima and Nagasaki.
Sullivan's remarks come after Putin delivered a rare televised address last week announcing partial military mobilization of the country's reservists and threatening to use nuclear weapons. The Russian leader baselessly accused Western countries of "nuclear blackmail" and said, without providing evidence, they are intentionally trying to provoke the Kremlin with escalated rhetoric surrounding their own nuclear weapons stockpiles.
"To those who allow themselves to make such statements about Russia, I would like to remind you that our country also has various means of destruction, and for some components more modern than those of the NATO countries," Putin said.
"And if the territorial integrity of our country is threatened, we will certainly use all the means at our disposal 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 that Putin could resort to the use of a nuclear weapon if he feels backed into a corner.
"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," CIA Director William Burns said in April.
Some Russia experts have expressed similar concerns, particularly as the Russian military struggles in the face of a Ukrainian counteroffensive. Retired Brig. Gen. Kevin Ryan, a former defense attaché to Russia, told Insider in mid-September he's now "even more concerned" about nuclear-weapon use by Putin.
But other Russia watchers are skeptical that Putin would use weapons of mass destruction.
Ivo Daalder, a former US ambassador to NATO, told Insider last Wednesday, "Anyone who finds it necessary to say that he's not bluffing most likely is."
Retired Adm. James Stavridis, a former supreme allied commander of NATO, told MSNBC on Friday that he's not losing "a lot of sleep" over Putin's nuclear threats. "I don't see Putin deciding to use a nuclear weapon," he said.