"We're still monitoring and watching this as closely as we can given President Putin's announcement yesterday," a senior US defense official said.

"I don't believe we've seen anything specific as a result of the direction that he gave," the official told reporters.

On Sunday, four days after launching the invasion of Ukraine, Putin announced that he had ordered his military chiefs "to put the deterrence forces of the Russian army into a special mode of combat service."

But Western officials say there has been no palpable change in the stance of Russia's powerful nuclear arsenal.

The US official called the move "unnecessary and very escalatory," insisting that Russia was facing no strategic threat from the West, and added that it was difficult to know what Putin's intentions were.

The US official said the language Putin used in his order was not consistent with what would be expected in mobilizing nuclear forces.

"It's not a doctrinal term, and the way he characterized it, a 'special combat duty alert' ... it's not a term of art in what we understand to be Russian doctrine," the official said.

Nor has the Pentagon seen Belarus troops take part in Russia's invasion of Ukraine, despite concerns that they will.

Belarus, which borders Ukraine and is a close ally of Russia, did allow its territory to be used as a staging ground for the invasion.

"We have seen no indications that Belarusian troops are being readied to move into Ukraine, and certainly no indications that they are in fact, moving or are in Ukraine," the official said.

The official also said there was no evidence Russia would place nuclear weapons in Belarus, as a way of challenging NATO's support for Ukraine.