President Joe Biden said Thursday that there is a "very high" risk of a Russian invasion of Ukraine and that could happen within "several days."

Speaking at the White House, Biden said the United States saw no signs of a claimed Russian withdrawal of forces along its border with Ukraine. He said the U.S. has "reason to believe" that Russia is "engaged in a false flag operation to have an excuse to go in."

He told reporters in Washington: "Every indication we have is they're prepared to go into Ukraine, attack Ukraine."

Earlier, a U.S. Embassy spokesman told a Russian news agency Thursday that Russia has expelled the deputy chief of of the U.S. mission in Moscow.

No details were given of why Bart Gorman was expelled. The State Department confirmed the expulsion, calling it unprovoked.

The move comes amid heightened tensions between Russia and the U.S., fuelled by fears that Moscow plans to invade Ukraine.

Spokesman Jason Rebholz told the state RIA Novosti news agency that Gorman was second in command in the U.S. Embassy in Moscow and had an open visa. He spent less than three years in Moscow, the report said.