The United States has dramatically raised the alarm over Ukraine, warning that a Russian invasion - starting with civilians caught under aerial bombing - could start in days.

An attack by the more than 100,000 Russian troops currently massed next to Ukraine "could occur any day now," White House National Security Advisor Jake Sullivan told reporters in Washington.

Dismissing speculation that the Kremlin would never trigger the crisis while the Beijing Olympics were still underway in close Russian ally China, Mr Sullivan said such an attack "could occur" before the Games end on 20 February.

The scenario of an imminent attack is "a very, very distinct possibility," he said.

A US official revealed that President Joe Biden will speak with Russian leader Vladimir Putin tomorrow, while the French government said that President Emmanuel Macron will also be calling the Russian leader.

Mr Sullivan stressed that it was not yet known whether President Putin had taken a decision to invade the Ukraine, saying "we can't predict the exact determination".

But he made clear that the US was bracing for the worst, including a "rapid assault" on the capital Kyiv.

"If a Russian attack on Ukraine proceeds, it is likely to begin with aerial bombing and missile attacks that could obviously kill civilians," he said. "Any American in Ukraine should leave as soon as possible, and in any event in the next 24 to 48 hours."

Mr Sullivan spoke shortly after Biden and six European leaders, the heads of NATO and the European Union held talks on the worst crisis between the West and Russia since the end of the Cold War.

Underlining the bleak outlook, a string of countries joined the exodus of diplomats and citizens from Ukraine, while oil prices surged and US equities tumbled.

Mr Sullivan repeated warnings that Russia risks severe Western sanctions and said that NATO, which Mr Putin wants to push back from eastern Europe, is now "more cohesive, more purposeful, more dynamic than any time in recent memory."

The Pentagon announced it was sending 3,000 more troops to bolster ally Poland.

The US has warned that a Russian invasion could come at any time

Following the group phone call between US and European leaders, German Chancellor Olaf Scholz's spokesman said "the aim is to prevent a war in Europe."

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But if Moscow fails to pull back, "the allies are determined to jointly take swift and deep sanctions against Russia."

These sanctions would target the financial and energy sectors, EU chief Ursula von der Leyen said.

NATO Secretary General Jens Stoltenberg has warned of the "real risk for a new armed conflict in Europe", while US Secretary of State Antony Blinken said a Russian invasion could come "any time".

Putting it bluntly in the call with western leaders, British Prime Minister Boris Johnson told them "he feared for the security of Europe", his Downing Street office said.

Britain and Norway have joined the US in asking their nationals to leave Ukraine.

There are no plans to withdraw staff from the Embassy of Ireland in Kyiv at this time, the Department of Foreign Affairs confirmed.

NATO Allies have for months raised the alarm over a possible invasion of Ukraine as tens of thousands of Russian troops mass along the border.

Russia has denied any plan to invade.

The US has warned Russia of unprecedented sanctions if its tanks roll into Ukraine, in particular promising an end to the controversial new Nord Stream 2 gas pipeline from Russia to Europe.