Biden to press Putin as fears of Ukraine war spiral

US president Joe Biden prepared to sound out Russia's Vladimir Putin on Saturday and Ukraine urged its citizens not to panic after Washington warned that an all-out invasion could begin "any day".

Weeks of tensions that have seen Russia surround its western neighbour with more than 100,000 troops intensified when the Kremlin launched its biggest naval drills in years across the Black Sea.

The exercises off the coast of Ukraine's Odessa added urgency to a hastily arranged call Saturday between Biden and Putin aimed at defusing one of the gravest crises in East-West relations since the Cold War.

Putin began his afternoon by holding talks with France's Emmanuel Macron that the French presidency said lasted 100 minutes.

No details were immediately released but the French leader has spearheaded EU efforts to ease fears of a major war breaking out in eastern Europe.

Russia on Saturday added to the ominous tone by pulling some of its diplomatic staff out of Ukraine.

The foreign ministry in Moscow said its decision was prompted by fears of "possible provocations from the Kyiv regime".

But Washington and a host of European countries cited the growing threat of a Russian invasion as they called on their citizens to leave Ukraine as soon possible.

Germany, Belgium and the Netherlands became the latest European countries to advise their citizens to leave Ukraine while the US embassy ordered "most" of its Kyiv staff to leave.

The prospect of frightened Westerners fleeing their country prompted Ukraine's foreign ministry to issue an appeal to its citizens to "remain calm".

"Right now, the people's biggest enemy is panic," Ukrainian president Volodymyr Zelensky said on a visit to troops stationed near the Russian-annexed peninsula of Crimea.

Washington on Friday issued its most dire warning yet that Russia had assembled enough forces to launch a serious assault.

"Our view that military action could occur any day now, and could occur before the end of the Olympics, is only growing in terms of its robustness," US National Security Advisor Jake Sullivan warned.

US military assessments had earlier said the Kremlin may want to wait for the Beijing Winter Olympic Games to end on 20 February before launching an offensive so as not to offend Russia's ally China.

Ukrainian leaders have been trying to talk down the prospects of an all-out war because of the damaging effect it was having on the country's teetering economy and public morale.

But the mood across the country remained tense.

The mayor's office of Kyiv announced that it had prepared an emergency evacuation plan for the capital's three million residents as a precaution.

Sullivan stopped short on Friday of saying that the United States has concluded that Putin has made the decision to attack.

But some US and German media cited intelligence sources and officials as saying that a war could begin at some point after Putin concludes talks with German Chancellor Olaf Scholz in Moscow on Tuesday.

The German leader is due to travel to Kyiv on Monday and then visit Putin as part of Europe's efforts to keep lines of communication open with Moscow.

Russia is demanding binding security guarantees from the West that includes a pledge to roll NATO forces out of eastern Europe and to never expand into Ukraine.

Washington has flatly rejected the demands while offering to discuss a new European disarmament agreement with Moscow.

Russia has called the US proposal woefully insufficient.

The diplomatic push continued on Saturday with a new round of inconclusive talks between US Secretary of State Antony Blinken and Russian foreign minister Sergei Lavrov.

The State Department said Blinken told Lavrov that diplomatic channels "remained open" but required "Moscow to deescalate and engage in good-faith discussions".

Lavrov countered that the West had ignored "key" Moscow demands and accused the United States of seeking to provoke a conflict in Ukraine, his ministry said.

Blinken said the United States was also still waiting for a response to "some of the ideas" floated by Washington.

Macron had said after a visit to Putin last Monday that he had secured a pledge of "no degradation nor escalation" from the Kremlin.

Sullivan repeated warnings that Russia risks severe Western sanctions and said that NATO 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.

European leaders also resolved to punish Russia with severe economic sanctions if it attacks.

"The aim is to prevent a war in Europe," Scholz's spokesman said after a call between US and European leaders.

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

Sullivan spoke to von der Leyen's chief of staff by video call to coordinate "the details of a potential transatlantic response, including both financial sanctions and export controls," the White House said.