Russia is preparing to fabricate a reason to invade Ukraine and launch an attack in the coming days, the US says.

On Thursday, President Joe Biden said military action could begin imminently, but stressed that a diplomatic solution was still possible.

His top foreign policy official later listed several ways Moscow could stage justification for an invasion.

Russia said the claims were "baseless" and accused the US of stoking tensions.

It has repeatedly denied any plan to invade its western neighbour.

Moscow insists it is moving troops away from the Ukrainian border, but that claim has been fiercely contested by Western powers.

"We have reason to believe they are engaged in a false flag operation to have an excuse to go in," Mr Biden told reporters outside the White House.