The United States said on Sunday (local time) that Russia could invade Ukraine "any day now" and might create a surprise pretext for an attack, as the German chancellor prepared for talks this week with President Vladimir Putin to try to ease the crisis.

Washington has said the door for diplomacy remained open but it has also repeatedly said Russia's military, which has more than 100,000 troops massed near Ukraine, was poised to act.

Moscow denies any such plans and has called comments "hysteria", but no breakthrough that could ease the crisis has yet emerged from high-level talks between top Russian and Western officials in recent days.

German Chancellor Olaf Scholz called for Russia to de-escalate on the eve of his trip that takes him to Kyiv on Monday and Moscow on Tuesday. A German official said Berlin did not expect "concrete results" but said diplomacy was important.

Scholz warned of sanctions if Moscow did invade.

"We cannot perfectly predict the day, but we have now been saying for some time that we are in the window, and an invasion could begin - a major military action could begin - by Russia in Ukraine any day now," White House National Security Adviser Jake Sullivan told CNN.

Sullivan said Washington would continue sharing intelligence with the world to deny Moscow the ability to stage a surprise "false flag" operation that could be a pretext for an attack.

U.S. officials said they could not confirm reports that U.S. intelligance indicated Russia planned to invade on Wednesday.

U.S. President Joe Biden, who is due to speak to his Ukrainian counterpart Volodymyr Zelenskiy on Sunday, told Putin in a call on Saturday the West would respond decisively to any invasion and such an attack would harm and isolate Moscow.

A senior U.S. administration official said Biden's call was 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.

Putin wants 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.

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

DIPLOMATIC PATH

"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," U.S. Secretary of State Anthony Blinken said after he held talks on Saturday with Asian allies.

Washington ordered most of its embassy staff on Saturday to leave Ukraine immediately. Its European allies and others have also been scaling back or evacuating staff from their Kyiv missions and have urged citizens to leave or avoid travel to Ukraine.

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.

Amid the tension, Dutch carrier KLM said it would stop flying to Ukraine and Germany's Lufthansa said it was considering suspending flights.

An adviser in Ukraine's presidency, Mykhailo Podolyak, said that regardless of what airlines chose to do Kyiv had no plans to close its airspace as such a move would resemble "a kind of partial blockade".

A French presidency official said on Saturday, after President Emmanuel Macron spoke with Putin, that there were no indications from what the Russian leader said that Russia was preparing an offensive.

But the official said Paris remained "extremely vigilant".

British defence minister Ben Wallace cautioned against putting too much hope in talks, saying there was "a whiff of Munich in the air from some in the West", referring to a 1938 pact that failed to halt German expansionism under Adolf Hitler.

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