KYIV: US President Joe Biden and France's Emmanuel Macron 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 revved up another notch 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.

The Russian leader is also due to speak later Saturday with Macron. The talks come after a week of frantic shuttle diplomacy by the French leader and European officials did little to ease fears of 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".

US says more Russian troops mass near Ukraine, invasion could come at any time

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 became the latest European country to advise its citizens to leave Ukraine while the US embassy in Kyiv ordered non-emergency staff to leave Ukraine.

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

"At the moment, it is critically important to remain calm, to consolidate inside the country, to avoid destabilising actions and those that sow panic," the ministry said.

"Ukrainian diplomats are in constant contact with all its key partners, swiftly receiving the information needed to prepare a well-timed response."