Ukraine could drop its bid to join NATO to avoid war with Russia, the BBC quoted the country's ambassador to Britain as saying, in what would amount to a major concession to Moscow in response to the build-up of Russian troops on its borders.

Ambassador Vadym Prystaiko told the BBC Ukraine was willing to be "flexible" over its goal to join the Atlantic military alliance, a move Russian President Vladimir Putin has said would be a trigger for war.

"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.

US says Russia may create pretext to attack Ukraine

Ukraine is not a NATO member but has a promise dating from 2008 that it will eventually be given the opportunity to join, a step that would bring the US-led alliance to Russia's border.

Putin says Ukraine's growing ties with the alliance could make it a launch pad for NATO missiles targeted at Russia. He says Russia needs to lay down "red lines " to prevent that.

Russia has moved more than 100,000 troops and heavy weapons to within striking distance of Ukraine in recent weeks, prompting the United States and its NATO allies to warn that an invasion could be imminent.

Moscow denies it is planning an attack, calling the military manoeuvres exercises, but is has issued written demands that NATO forgo any further expansion eastwards including Ukraine. NATO members have rejected the demand.

US President Joe Biden spoke to his Ukrainian counterpart Volodymyr Zelenskiy on Sunday and agreed to continue diplomatic efforts to try to resolve the crisis, the White House said.