The US is withdrawing nearly all of its remaining soldiers from Ukraine, the Pentagon announced on Saturday, as tensions soar over a possible Russian invasion of the eastern European country.

Defence Secretary Lloyd Austin “has ordered the temporary repositioning of the 160 members of the Florida National Guard” who were in the country, “advising and mentoring Ukrainian forces,” Pentagon spokesperson John Kirby said in a statement.

The troops — who Kirby said were being withdrawn “with the safety and security of our personnel foremost in mind” — will be repositioned “elsewhere in Europe.”

Since 2015, reservists from the US National Guard have been advising and training Ukraine's army alongside soldiers from other North Atlantic Treaty Organisation (Nato) countries, notably Canada and Germany.

“This repositioning does not signify a change in our determination to support Ukraine's Armed Forces, but will provide flexibility in assuring allies and deterring aggression,” Kirby said.

Putin-Biden phone call

Meanwhile, Russian President Vladimir Putin and US President Joe Biden held a high-stakes telephone call as a tense world watched and worried that an invasion of Ukraine could begin within days.

The closely watched call between Biden and Putin began at 11:04 am EST, the White House said. Biden conducted the call from Camp David.

Before talking to Biden, Putin had a telephone call with French President Emmanuel Macron, who met with him in Moscow earlier in the week to try to resolve the biggest security crisis between Russia and the West since the Cold War. A Kremlin summary of the call suggested that little progress was made toward cooling down the tensions.

Earlier today, the US ordered all non-emergency Kyiv embassy staff to leave the country because of the threat of a Russian invasion.

“Despite the reduction in diplomatic staff, the core embassy team, our dedicated Ukrainian colleagues, and State Department and US personnel around the world will continue relentless diplomatic and assistance efforts in support of Ukraine's security, democracy, and prosperity,” the US embassy in Kyiv tweeted after the development.

Warning of attack

The US had dramatically raised the alarm over Ukraine on Friday, saying a Russian invasion starting with civilians caught under aerial bombing could begin in days and telling US citizens to leave within 48 hours.

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, Sullivan said such an attack “could occur” before the Games end on February 20.

The scenario of an imminent attack is “a very, very distinct possibility”, Sullivan added.

While stressing that it was not yet known whether Putin had taken a decision, saying “we can't predict the exact determination,” Sullivan made clear the United States 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.”

Sullivan spoke shortly after President Joe 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.

Western, Nato unity

Sullivan repeated warnings that Russia risks severe Western sanctions and said that Nato, which 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.

And US Secretary of State Antony Blinken told his Ukrainian counterpart Dmytro Kuleba in a phone call that “Ukraine continues to have the United States' enduring and steadfast support for its sovereignty and territorial integrity,” the State Department said.

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

Sullivan spoke to der Leyen's chief of staff Bjoern Seibert by video call on Friday to coordinate “the details of a potential transatlantic response, including both financial sanctions and export controls”, according to a White House statement.

Russia surrounding Ukraine

Russian naval forces and troops, including units brought in from all over the vast country, now surround Ukraine to the south, east and north.

Russia, which denies any plan to attack Ukraine, already controls the Crimea territory seized in 2014 and supports separatist forces controlling Ukraine's Donbas region in the east.

The Kremlin says its goal is to get Nato to agree to never give Ukraine membership and also to withdraw from eastern European countries already in the alliance, effectively carving Europe into Cold War-style spheres of influence.

The United States and its European allies reject the demands, insisting that Nato poses no threat to Russia.

Read: Nato and Russia lay out stark differences on Ukraine crisis

Adding to tensions, large-scale Russian military drills were underway on Friday with authoritarian ally Belarus, which lies just north of Kyiv and also borders the European Union.

Russia's defence ministry said on Friday it was also holding military exercises near Ukraine's border in the Black Sea.

According to the head of Norway's military intelligence service, Russia is operationally ready to conduct a wide range of military operations in Ukraine and the Kremlin just needs to make the call.

The top US general and his Russian counterpart talked on Friday by phone, the Pentagon said, giving no details of the discussion.

And the European Union said its non-essential staff should leave Ukraine, while Israel said families of its diplomats were being pulled out. Norway joined Britain in telling its nationals to leave.

Shuttle diplomacy

The growing alarm comes despite efforts at shuttle diplomacy by European officials.

Macron visited both Moscow and Kyiv earlier this week and Scholz is expected to do the same in the coming days. Scholz will also hold his first in-person meeting with Putin in Moscow.

British Defence Secretary Ben Wallace was in Moscow on Friday for rare talks with his Russian counterpart Sergei Shoigu.

He was accompanied by the UK's Chief of Defence Staff Tony Radakin, and the pair will also meet Russia's top army general Valery Gerasimov.