"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.

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 US 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."