WASHINGTON: US President Joe Biden on Thursday said the threat of a Russian invasion of Ukraine was "very high" and could take place within days, despite Moscow's claim to be pulling troops from the border.

The threat is "very high, because they have not moved any of their troops out. They've moved more troops in," Biden told reporters at the White House. "We have reason to believe they're engaged in a false flag operation to have an excuse to go in."

"Every indication we have is that they're prepared to go into Ukraine, attack Ukraine," he said. "My sense is it will happen in the next several days."

US, Nato say Russia is building up troops near Ukraine

Biden said he had not yet read a new, written response from Russian President Vladimir Putin to US proposals for a diplomatic way out of the impasse.

Russia's military has surrounded much of Ukraine's borders as part of a bid to overturn the country's Western-orientated policies, including the longterm goal of joining NATO.

Biden said there is still "a diplomatic path" and that Secretary of State Antony Blinken would "lay out what that path is" in a speech at the United Nations on Thursday.

However, Biden said "I've no plans to call to Putin."