Officials also said that Russian troops in Ukraine continue to move ahead slowly, hampered by fuel supply issues, and downplayed reports that the Belarus army intended to enter Ukraine as reinforcements for Moscow's invasion force.

On Sunday, four days after Moscow launched the invasion, 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."

Officials at the US State and Defense Departments said they were still trying to determine just what action was being taken in response to the order.

"We think provocative rhetoric regarding nuclear weapons is dangerous, adds to the risk of miscalculation, should be avoided, and we will not indulge in it," a State Department spokesperson said.

"Russia and the United States have long agreed that nuclear use would have devastating consequences, and have stated many times, including earlier this year, that a nuclear war cannot be won, and must never be fought," the spokesperson said.

A senior Pentagon official said there had been no palpable change in the stance of Russia's powerful nuclear arsenal, but called Putin's order "unnecessary and very escalatory."

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

The US officials said Russia was facing no military threat from the West, even as the United States and NATO allies were supplying arms to Ukraine to counter the Russian invasion.

The Pentagon 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.

- Resistance 'pretty effective'-

The Pentagon official said that the Russian invasion force continued to wrestle with logistical problems that had left them short of their apparent goals Monday.

The main Russian targets remain to capture Kharkiv, the site of the heaviest fighting, and Kyiv, the capital.

And while satellite photos from Maxar show a massive Russian military column north of Kyiv, they remain 25 kilometers away, advancing just five kilometers in the past day, according to the official.

"They're running out of gas, and they're having logistics problems," the official said.

"The Ukrainians have proved pretty effective" in their resistance, the official added.

Meanwhile the official said the Pentagon had not 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.