WASHINGTON: President Vladimir Putin's nuclear forces alert represents a "totally unacceptable" escalation of the invasion of Ukraine, the United States said Sunday as it accused the Russian leader of fabricating threats to justify "further aggression."

"This is a pattern that we've seen from President Putin through the course of this conflict, which is manufacturing threats that don't exist in order to justify further aggression," White House press secretary Jen Psaki said on ABC when asked about the announcement from Moscow.

At the Pentagon, a senior official called the move "escalatory" but would not say if US nuclear forces had changed their stance in response.

Speaking to journalists, the defense official said the US had no reason to doubt that Putin's announcement was genuine, but had no information yet as to how it would be implemented.

"We believe that this is not only an unnecessary step for him to take, but an escalatory one," the official said, on grounds of anonymity.

"Unnecessary because Russia has never been under threat by the West, by NATO and certainly wasn't under any threat by Ukraine," the official said.

Putin orders nuclear forces on high alert

"And escalatory because it is clearly potentially putting in play forces that, if there's a miscalculation, could make things much, much more dangerous."

The American ambassador to the United Nations, Linda Thomas-Greenfield, said she condemned Putin's step in the strongest terms.

"It means that President Putin is continuing to escalate this war in a manner that is totally unacceptable," Thomas-Greenfield said in an interview on CBS.

Moscow has the world's second-largest arsenal of nuclear weapons and a huge cache of ballistic missiles which form the backbone of the country's deterrence forces.

Putin said he was putting Russia's nuclear forces on high alert because, he said, Western countries are taking "unfriendly" steps against his country in the wake of the Russian invasion of Ukraine Thursday.

The US defense official declined to say whether US nuclear forces, the world's largest, had been placed on higher alert.

"We do not talk about the specifics of our strategic deterrence posture," the official said.

"I would just tell you that we remain confident in our ability to defend ourselves and our allies and our partners. And that includes in the strategic deterrence realm."