WASHINGTON/KYIV: 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.

US defends warnings in standoff with Russia over Ukraine

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,” White House National Security Adviser Jake 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.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 1,500 tonnes of ammunition from allies delivered on 17 flights, including about 180 tonnes from the United States. Sullivan repeated a warning for Americans to leave Ukraine, after the State Department said the US embassy’s remaining diplomatic staff would work from the western city of Lviv.