Reuters - Russia could invade Ukraine at any time and might create a surprise pretext for an attack, the United States said on Sunday, as it reaffirmed a pledge to defend "every inch" of NATO territory.

Russia has more than 100,000 troops massed near Ukraine, which is not part of the Atlantic military alliance, and Washington - while keeping open the diplomatic channels that have so far failed to ease the crisis - 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.

A German official said Berlin did not expect "concrete results" but diplomacy was important.

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.

U.S. officials said they could not confirm reports that U.S. 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 added in a separate CBS interview.

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, the White House said after the call.

Zelenskiy's office said he invited Biden to visit Ukraine soon. The White House declined to comment.

Agreeing with the U.S. assessment that an invasion could happen "at any moment," a British government spokesperson said Britain was working on a package of military support and economic aid for Ukraine to be announced in coming days. Prime Minister Boris Johnson will make a trip to Europe later this week to build support to end the standoff with Russia.