US President Joe Biden and Russia’s Vladimir Putin spoke by phone on Saturday after Washington and its allies warned that Russian forces could invade Ukraine at any moment.

The US state department ordered most of its embassy staff to leave Ukraine, adding to its call on Friday for private citizens to get out of the country within 48 hours. The Pentagon said it was withdrawing about 150 military trainers.

In the latest effort to avert hostilities, the Biden-Putin call began at 1604 GMT, a White House official said.

Russia’s military build-up near Ukraine and a surge of military activity has fueled fears that Russia could invade. Russia denies having any such plans.

Earlier on Saturday, French President Emmanuel Macron told Putin that sincere negotiations were incompatible with an escalation in tensions over Ukraine, France said.

Biden and Macron are due to speak after their separate calls with Putin, according to a French presidency official. The official said there were no indications from what Putin told Macron that Russia is preparing an offensive against Ukraine.

“We are nevertheless extremely vigilant and alert to the Russian (military) posture in order to avoid the worst,” the official said.

Washington said on Friday that a Russian invasion of Ukraine, likely beginning with an air assault, could occur at any time. Portugal, Australia, New Zealand, Germany, Italy, the Netherlands and other countries have also urged their citizens to leave Ukraine.

Moscow has repeatedly disputed Washington’s version of events, saying it has massed more than 100,000 troops near the Ukrainian border to maintain its own security against aggression by Nato allies.

Russia, which has accused western nations of spreading lies to distract from their own acts, meanwhile said on Saturday that it had decided to “optimise” its diplomatic staff numbers in Ukraine, fearing “provocations” by Kiev or others. It said its embassy and consulates in Ukraine continued to perform their key functions.

US secretary of state Antony Blinken said Washington would impose swift economic sanctions if Putin decides to invade.

“I continue to hope that he will not choose the path of renewed aggression and he’ll chose the path of diplomacy and dialogue,” Blinken told reporters after a meeting with Pacific leaders in Fiji. “But if he doesn’t, we’re prepared.”

In a call with Blinken, Russia’s top diplomat Sergei Lavrov accused the US and its allies of waging a “propaganda campaign” about Russian aggression.