DONETSK: The United States said the diplomatic path remained open to end a standoff with Moscow over Ukraine but said the risk of Russian military action was high enough to warrant pulling U.S. embassy staff out of Kyiv.

US Secretary of State Antony Blinken was speaking after talks on Saturday with Japanese and South Korean counterparts, following Washington's warning that Russia's military, which has more than 100,000 troops massed near Ukraine, could invade at any moment.

Moscow, which has repeatedly denied it plans to invade and said it is responding to aggression by NATO allies, has dismissed those warnings as "hysteria".

"The diplomatic path remains open. The way for Moscow to show that it wants to pursue that path is simple. It should de-escalate, rather than escalate," Blinken said after his meetings in the U.S. Pacific archipelago of Hawaii.

In an hour-long call on Saturday, U.S. President Joe Biden told Russia's Vladimir Putin that the West would respond decisively to any invasion of Ukraine, adding such a step would produce widespread suffering and isolate Moscow.

Neither side said there had been any breakthroughs. A senior Biden administration official said the call was professional and substantive, but that there was no fundamental change.

The Kremlin said Putin told Biden that Washington had failed to take Russia's main concerns into account and it had received no "substantial answer" on key elements of its security demands.

Washington ordered most of its embassy staff on Saturday to leave Ukraine immediately due to the threat of an invasion.

"We ordered the departure of most of the Americans still at the U.S. embassy in Kyiv. The risk of Russian military action is high enough and the threat is imminent enough that this is the prudent thing to do," Blinken said in Honolulu.

Many of Washington's European allies and other countries have also been scaling back or evacuating staff from their Kyiv missions and have urged citizens to leave or avoid travel to Ukraine.

LEAVING DONETSK

U.S. staff at the Organisation for Security and Cooperation in Europe (OSCE) began leaving by car from the rebel-held city of Donetsk in east Ukraine on Sunday, a Reuters witness said.

The OSCE conducts operations in Ukraine including a civilian monitoring mission in Russian-backed, self-proclaimed separatist republics in the regions of Donetsk and Luhansk, where a war that erupted in 2014 has killed more than 14,000 people.

Australia said on Sunday it was evacuating its embassy in Kyiv and Prime Minister Scott Morrison called on China to not remain "chillingly silent" on the crisis.

President Xi Jinping hosted Putin at the opening day of the Winter Olympics this month and the two declared a "no limits" partnership, backing each other over standoffs on Ukraine and Taiwan with a promise to collaborate more against the West.

Putin is seeking security guarantees from the United States and NATO that include blocking Ukraine's entry into NATO; refraining from missile deployments near Russia's borders; and scaling back NATO's military infrastructure in Europe to 1997 levels.

Washington regards many of the proposals as non-starters but has pushed the Kremlin to discuss them jointly with Washington and its European allies.

The White House said Biden was due to compare notes with French President Emmanuel Macron, who spoke with Putin and Ukrainian President Volodymyr Zelenskiy on Saturday.

After Macron's call with Putin, a French presidency 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 French official said.

Amid Western diplomatic efforts, British defence secretary Ben Wallace cautioned against putting too much hope in talks.

"It may be that he (Putin) just switches off his tanks and we all go home but there is a whiff of Munich in the air from some in the West," Wallace told the Sunday Times of London, referring to a 1938 pact that gave Adolf Hitler land in the former Czechoslovakia as part of a failed attempt to persuade him to abandon territorial expansion.

"The worrying thing is that, despite the massive amount of increased diplomacy, that military build-up has continued."