US warns of 'massive' consequences if Russia attacks Ukraine

Caution comes as tension over Russia's deployment of some 100,000 troops along Ukrainian border fuels fears of invasion.

A Russian serviceman takes part in drills in the Rostov region in southern Russia near the border with Ukraine this week. AP

A top US official on Friday warned Russia against any new aggression against Ukraine and suggested demands from Moscow that Nato to stop expanding eastward were a non-starter.

The caution comes as tension over Russia's deployment of some 100,000 troops along its border with Ukraine fuels western fears of invasion.

“If there is any further aggression against Ukraine, that will have massive, massive consequences and will carry a high price,” a senior administration official said in a call with reporters.

Washington remains “gravely concerned with the large and unprovoked Russian build-up” on Ukraine's border, the official added.

Russia has denied it is planning to invade Ukraine and says its troop build-up comes in response to perceived threats to its own national security.

Moscow on Friday released two draft texts that made a string of sweeping demands including that Nato not expand eastward and end drills near Russia's borders.

“There are some things in the documents that the Russians know will be unacceptable. … But there are other things that we are prepared to work with,” the US official said.

Russia's demand that Nato not add any new members to its alliance appeared to be non-negotiable.

“We will not compromise on key principles on which European security is built, including … that all countries have the right to decide their own future and their own foreign policy, free from outside interference,” the official said.

“That goes for Ukraine and it also goes for Nato allies and the alliance itself with regard to how it provides collective defence for its members.”

Still, in a more positive diplomatic move, the official said a Christmas ceasefire in the Donbass region of eastern Ukraine is under consideration.

Moscow and Kiev have long accused each other of failing to enact a peace deal over Donbass, where more than 14,000 people are estimated to have been killed in clashes between the Ukrainian government and Russian-backed separatist forces since 2014.

“We are particularly interested … in seeing a Christmas ceasefire and a prisoner exchange. That's something that's under discussion,” the official said.

The US and Nato have repeatedly warned Russia of harsh consequences if it invades Ukraine, including tough economic sanctions and the cancellation of the Nord Stream 2 pipeline, which will carry Russian gas supplies to Germany, bypassing Ukraine.

“The Russian people don't need a war with Ukraine,” the US official said.

“They don't need their sons coming home in body bags. They don't need another foreign adventure.

“We hope that President [Vladimir] Putin will take this opportunity for diplomacy and will also listen to the needs of his own people.”