More Russian troops mass near Ukraine, invasion could come at any time: US

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, Washington said on Friday.

Moscow, for its part, ramped up its truculent response towards Western diplomacy, saying answers sent this week by the EU and NATO to its security demands showed "disrespect".

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

"Things could go crazy quickly," Biden told NBC News. Biden was due to hold a telephone summit on Friday to discuss the crisis with the leaders of Britain, Canada, France, Germany, Poland and Romania, as well as heads of NATO and the EU.

Secretary of State Antony Blinken, visiting Australia, said: "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 20 Feb.

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

With alarm spreading, Japan and the Netherlands also told their citizens on Friday to leave Ukraine immediately. The Dutch diplomatic mission would be pulled from Kyiv and moved far from the Russian frontier to Lviv in Ukraine's west.

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 responses this week on behalf of their member states, which they said had agreed to speak as one.

Russia's foreign ministry said on Friday it wanted individual answers 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."

Britain's defence secretary Ben Wallace was in Moscow on Friday, where defence minister Sergei Shoigu told him Russia would soon respond to the NATO and EU letters over its demands.

Paris and Kyiv said the Russian delegation demanded Ukraine negotiate directly with the separatists, a "red line" Ukraine has rejected since the conflict began in 2014.

US-based Maxar Technologies, which has been tracking the buildup of Russian forces, said images taken on Wednesday and Thursday showed large new deployments of troops, vehicles and warplanes at several locations in western Russia, Belarus and Crimea, which Russia annexed in 2014. The images could not be independently verified by Reuters.

Russia says it has the right to move forces around on its territory as it sees fit, and they pose no external threat.

Western countries have mostly stood together in threatening economic sanctions against Russia if it invades Ukraine, but have given conflicting views on the threat's immediacy.

Washington and London have warned an invasion could come within days. British prime minister Boris Johnson called the coming days the most dangerous moment in Europe's biggest security crisis for decades.

France's Macron, by contrast, has said he thinks Russia does not have designs on Ukraine but wants changes to European security arrangements, and called the existing Franco-German-led peace process for Ukraine's separatist conflict a way out.

Whatever its intentions, Moscow has responded dismissively to Western pressure. Pictures of Macron seated far from Putin at the opposite end of a huge table in the Kremlin went viral on the internet this week, widely mocked.