Russia has more than 100,000 troops massed near Ukraine, which is not part of the Atlantic military alliance, and Washington has repeatedly said an invasion is imminent.
Moscow denies any such plans and has accused the West of "hysteria".
German Chancellor Olaf Scholz, on the eve of a trip that takes him to Kyiv on Monday and Moscow for talks with President Vladimir Putin on Tuesday, called for Russia to de-escalate and warned of sanctions if Moscow did invade.
In what could amount to a major concession to Moscow, Ukraine's ambassador to Britain told the BBC Kyiv could drop its bid to join NATO to avoid war.
Ambassador Vadym Prystaiko was quoted as saying Ukraine was willing to be "flexible" over its goal to join the Atlantic military alliance.
"We might - especially being threatened like that, blackmailed by that, and pushed to it," Prystaiko was quoted as saying when asked if Kyiv could change its position on NATO membership.
In Washington, President Joe Biden's National Security Adviser Jake Sullivan said an invasion could begin "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.
US officials said they could not confirm reports that US intelligence indicated Russia planned to invade on Wednesday.
Sullivan said Washington would continue sharing what it learned with the world in order to deny Moscow the chance to stage a surprise "false flag" operation that could be a pretext for an attack.
It would also "defend every inch of NATO territory ... and Russia we think fully understands that message," Sullivan said.
Biden spoke to his Ukrainian counterpart Volodymyr Zelenskiy on Sunday and they agreed on the importance of continuing to pursue diplomacy and deterrence in response to Russia's military build-up.
Biden told Putin in a phone call on Saturday that the West would respond decisively to any invasion and such an attack would harm and isolate Moscow.
Ukrainian Defence Minister Oleksii Reznikov said on Twitter that Kyiv had so far received almost 1500 tonnes of ammunition from allies delivered on 17 flights, including about 180 tonnes from the US.
The Kremlin said Putin told Biden during their call on Saturday that Washington had failed to take Russia's main concerns into account, and that 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.
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 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 and others have been scaling back or evacuating embassy staff and urging citizens to depart immediately or avoid travel to Ukraine.
US 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 began in 2014 has killed more than 14,000 people.
Ukraine said on Sunday it wanted talks with Russia and members of the OSCE within 48 hours to discuss Russia's military build-up.
