US President Joe Biden will speak to Vladimir Putin on Saturday after the United States warned that a Russian invasion of Ukraine could begin in days.

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 Russian President Vladimir 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.

A US official said Biden would speak with Putin on Saturday, while the French government said that President Emmanuel Macron would also be calling the Russian leader on Saturday.

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.

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

Russia's defense ministry said 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 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 Defense Secretary Ben Wallace was in Moscow Friday for rare talks with his Russian counterpart Sergei Shoigu. 

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

Related Story