US President Joe Biden said Thursday that Russia could invade Ukraine "in a manner of several days" but a diplomatic resolution is still possible.

The threat of an invasion is "very high” and Moscow is moving troops and may be engaged in a false flag operation, Biden told reporters at the White House before his trip to the state of Ohio.

When asked if he thinks Russia will go through with an invasion, Biden replied: "Yes, I do. Not now….in a manner of several days".

The US president said he was awaiting a response to his letter sent to Russian President Vladimir Putin.

A senior Biden administration official said Wednesday that Russia has moved up to 7,000 forces to the Ukrainian border, countering claims by Moscow that it was pulling back some troops.