Russia has massed enough troops near Ukraine to launch a major invasion, Washington said on Friday, as it urged all US citizens to leave the country within 48 hours.

A Russian attack could begin any day and would likely start with an air assault, while a rapid advance on Kyiv was also possible, White House national security adviser Jake Sullivan told a media briefing.

It remains unclear, Sullivan said, whether Russian President Vladimir Putin had definitively given an order to start an invasion. Sullivan said he expected US President Joe Biden to seek out a phone call with Putin soon on the crisis.

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Earlier, as Moscow further stiffened its response to Western diplomacy, commercial satellite images from a private US company showed new Russian military deployments at several sites near the border.

After telling NBC News that things in Ukraine "could go crazy quickly," Biden held a phone call on the crisis with the leaders of Britain, Canada, France, Germany, Poland and Romania, as well as the heads of NATO and the EU.

Following that meeting and with alarm spreading, British Prime Minister Boris Johnson joined a handful of other nations in urging their citizens to leave Ukraine.

Johnson told the meeting that he feared for the security of Europe and stressed the need for "a heavy package of economic sanctions ready to go, should Russia make the devastating and destructive decision to invade Ukraine," his office said.

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Moscow, meanwhile, said answers sent this week by the EU and NATO to its security demands showed "disrespect".

Biden met his national security advisers overnight, a source familiar with the meeting said. US officials believed the crisis could be reaching a critical point, with rhetoric from Moscow hardening, six Russian warships reaching the Black Sea, and more Russian military equipment arriving in Belarus, the source said.

"We're in a window when an invasion could begin at any time, and to be clear, that includes during the Olympics," said Secretary of State Antony Blinken. 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."

Russia has denied it plans to invade Ukraine.

Stocks extended declines in the late afternoon, with the S&P 500 index recently down around 1.3%. The dollar, often seen as a destination for nervous investors, was recently up 0.2% against a basket of its peers.

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First Published:Feb 12, 2022 10:16 AM IST