At Monday's UN Security Council meeting, Russian Ambassador Vasily Nebenzya said there was no proof that Russia was planning military action against Ukraine, and that its troop build-up was not confirmed by the UN.

He said Russia often deployed troops on its own territory and that this was none of Washington's business.

Russia had tried to block the open session of the UN body but was outvoted by 10 votes to two.

The Biden administration was "whipping up tensions and rhetoric, and provoking escalation", Mr Nebenzya said.

"This isn't just unacceptable interference in the internal affairs of our state, it's also an attempt to mislead the international community about the true situation in the region and the reasons for the current global tensions," he said.

Ms Thomas-Greenfield said the US continued to believe there was a diplomatic solution but warned that the US would act decisively if Russia invaded Ukraine, the consequences of which would be "horrific".

"This is the largest... mobilisation of troops in Europe in decades," she said.

"And as we speak, Russia is sending even more forces and arms to join them."

Moscow was planning to increase its force deployed in neighbouring Belarus, on Ukraine's northern border, to 30,000, she added.

Late on Monday, the US ordered the departure of family members of American government employees from Belarus, citing the "unusual and concerning Russian military buildup". A similar order was earlier issued to families of US government personnel in the American embassy in Ukraine's capital Kyiv.