Russia is massing yet more troops near Ukraine and an invasion could come at any time, perhaps before the end of this month’s Winter Olympics, US secretary of state Antony Blinken said on Friday.

Moscow, for its part, ramped up its truculent response towards a western diplomatic push to defuse the crisis, dismissing answers sent this week by the EU and Nato to its security demands as an insult.

Commercial satellite images published by a private US company showed new Russian military deployments at several locations near Ukraine.

In his starkest warning yet to Americans in Ukraine to get out now, President Joe Biden said he would not send troops to rescue US citizens in the event of a Russian assault.

“Things could go crazy quickly,” Biden told NBC News.

Blinken, visiting Australia, told a news conference: “We’re in a window when an invasion could begin at any time, and to be clear, that includes during the Olympics.”

The Beijing games end on February 20.

“Simply put, we continue to see very troubling signs of Russian escalation, including new forces arriving at the Ukrainian border,” Blinken said.

Russia has already massed more than 100,000 troops near Ukraine, and this week it launched joint military exercises in neighbouring Belarus and naval drills in the Black Sea.

Moscow denies plans to invade Ukraine, but says it could take unspecified “military-technical” action unless a series of demands are met, including promises from Nato never to admit Ukraine and to withdraw forces from eastern Europe.

The West has said those main demands are non-starters. The EU and Nato alliance delivered joint responses this week, saying their member states had agreed to speak as one. Russia's Foreign Ministry said on Friday it had demanded an individual answer from each country, and called the collective response insulting.

“Such a step cannot be seen as anything other than a sign of diplomatic impoliteness and disrespect for our request,” the ministry said.

Several western countries launched diplomatic pushes this week to persuade Russia to back down, but Moscow brushed them off, yielding no concessions to French President Emmanuel Macron who visited on Monday and openly mocking British foreign secretary Liz Truss who came on Thursday.

Four-way talks in Berlin between Russia, Ukraine, Germany and France, part of a longstanding peace process in a conflict between Ukraine and Russian-backed separatists, also yielded no progress.