"Russia continues escalating and has not reversed its troop buildup," a US State Department spokesperson told AFP.

Washington and its allies were "closely" monitoring the situation, the spokesperson said, repeating warnings that "any further aggression against Ukraine would have massive consequences and would carry a severe price."

The statement came as Germany also reported "further troop movements on the border" between Russia and Ukraine.

But White House Press Secretary Jen Psaki emphasized "an open line of diplomatic discussion and engagement that is happening and that we expect to continue, that we hope to continue."

The West accuses Moscow of having massed tens of thousands of soldiers at the border to prepare a possible invasion of Ukraine.

Russia denies plotting an invasion and has demanded legal guarantees from the United States and NATO, demanding the alliance stop an eastward expansion.

The United States and its allies have already warned of unprecedented sanctions if Russia invades.

"We strongly urge Russia to de-escalate by pulling back troops from its border with Ukraine," the State Department spokesperson added.

"Our goal is de-escalation through diplomacy; the US is ready to engage in diplomacy in January through multiple channels."

The combined threats and diplomatic appeals have not stopped Russian President Vladimir Putin from adopting a particularly vehement tone this week.

He warned Tuesday that Moscow was prepared to take military steps in response to "unfriendly" Western actions over Ukraine, in a sharp escalation of rhetoric.

"President Putin has his own audience. It is not the United States of America," Psaki said Wednesday.

"NATO is a defensive alliance. We don't have aggressive intent with Russia," she continued.

"The aggression we've seen at the Ukrainian border, that bellicose rhetoric has been coming from one side (...) and I think anybody can see that pretty clear as day."