According to the US, Russia may devise a surprise justification for an assault on Ukraine.
The US stated on Sunday that Russia might invade Ukraine at any time and may construct a surprise pretext for an attack, reiterating its commitment to protecting "every inch" of NATO territory.
Russia has more than 100,000 troops stationed near Ukraine, which is not a member of the Atlantic military alliance, and Washington has frequently warned that an invasion is inevitable, despite leaving diplomatic channels open that have so far failed to resolve the problem.
Moscow has denied any such preparations, accusing the West of being "hysteria."
On the eve of a trip to Kyiv on Monday and Moscow for discussions with President Vladimir Putin on Tuesday, German Chancellor Olaf Scholz called on Russia to de-escalate and threatened consequences if Moscow invaded.
Berlin does not expect "concrete results," according to a German source, but diplomacy is crucial.
Ukraine's ambassador to the UK told the BBC that, in order to avert war, Kyiv should renounce its desire to join NATO. This could amount to a huge surrender to Moscow.
Ukraine's ambassador, Vadym Prystaiko, was quoted as suggesting that the country was willing to be "flexible" in its pursuit of membership in the Atlantic military alliance.
"We might - especially being threatened like that, blackmailed by that, and pushed to it," When asked if Kyiv may change its mind about NATO membership, Prystaiko was quoted as saying.
President Joe Biden's National Security Adviser, Jake Sullivan, stated in Washington that an invasion might start "any day now."
"We cannot perfectly predict the day, but we have now been saying for some time that we are in the window," Sullivan told CNN.
Officials from the United States said they couldn't confirm claims that Russian intelligence was planning an invasion on Wednesday.
According to Sullivan, Washington will continue to share what it has learned with the rest of the world in order to prevent Moscow from staging a surprise "false flag" operation that may be used as a justification for a strike.
It would also "defend every inch of NATO territory ... and Russia we think fully understands that message," Sullivan added in a separate CBS interview.
According to the White House, Biden spoke with his Ukrainian colleague Volodymyr Zelenskiy on Sunday, and the two agreed on the significance of continuing to pursue diplomacy and deterrence in response to Russia's military build-up.
According to Zelenskiy's office, Biden has been asked to visit Ukraine soon. The White House did not respond to a request for comment.
A British government official said Britain was working on a package of military and economic aid for Ukraine that would be published in the coming days, agreeing with the US assessment that an invasion might come "at any moment." Boris Johnson, the British Prime Minister, will travel to Europe later this week to drum up support for ending the deadlock with Russia.
In a phone chat with Putin on Saturday, Biden stated that any invasion would be met with retaliation from the West and that such an attack would hurt and isolate Moscow.
On Twitter, Ukrainian Defense Minister Oleksii Reznikov said Kyiv has received about 1,500 tonnes of weapons from allies on 17 planes so far, including roughly 180 tonnes from the US.
Canada's defence minister announced that its military soldiers stationed in Ukraine have been temporarily relocated to an undisclosed location in Europe. Since 2015, Canada has maintained a 200-strong training mission in western Ukraine, which is home to the world's third-largest Ukrainian population after Ukraine and Russia.
RUSSIAN SECURITY DEMANDS
Putin told Biden on Saturday that Washington had failed to address Russia's core concerns and that it had received no "substantial answer" on key components of its security requests, according to the Kremlin.
Putin is seeking guarantees from the US and NATO, including delaying Ukraine's admission to NATO, refraining from missile deployments near Russia's borders, and reducing NATO's military infrastructure in Europe to 1997 levels.
Many of the recommendations are non-starters for Washington, but it has pushed the Kremlin to consider them with Washington and its European allies.
"The diplomatic path remains open. The way for Moscow to show that it wants to pursue that path is simple," US Secretary of State Anthony Blinken said after he held talks on Saturday with Asian allies.
Washington and its European allies, among others, have begun reducing or withdrawing embassy staff and advising nationals to leave Ukraine as soon as possible.
According to a Reuters witness, US workers at the Organisation for Security and Cooperation in Europe (OSCE) began departing the rebel-held city of Donetsk in east Ukraine by car on Sunday.
In Ukraine, the OSCE runs operations, including a civilian monitoring mission in Russian-backed, self-proclaimed separatist republics in the Donetsk and Luhansk areas, where a war that began in 2014 has killed over 14,000 people.
On Sunday, Ukraine said it wants to meet with Russia and OSCE members within 48 hours to discuss Russia's military build-up. After Kyiv invoked a section of the Vienna Document, a collection of security agreements, on Friday to demand Moscow explain its military activity, Ukrainian Foreign Minister Dmytro Kuleba said Moscow had not responded.
KLM, a Dutch airline, said it would stop flying to Ukraine, while Lufthansa, a German airline, indicated it was considering doing so.
Mykhailo Podolyak, a Zelenskiy adviser, warned that regardless of what airlines do, Kyiv will not close its airspace since that would be "a kind of partial blockade."
After President Emmanuel Macron spoke with Putin on Saturday, a French administration official said there were no indications from what Putin said that Moscow was planning an offensive, though Paris remained "extremely vigilant."
Ben Wallace, the British defence minister, warned against placing too much faith in the negotiations, telling The Sunday Times of London that there was "a whiff of Munich in the air from some in the West," referring to a 1938 deal that failed to stop Adolf Hitler's German expansionism.