WASHINGTON, Jan. 20 (REUTERS) -- US President Joe Biden said on Thursday (Jan 20) that he has made clear to Russian President Vladimir Putin that any Russian movement into Ukraine would be considered an invasion, after he earlier suggested allies were split over how to react to any "minor incursion".

"I have been absolutely clear with President Putin, he has no misunderstanding. If any, any assembled Russian units move across the Ukrainian border that is an invasion," Biden told reporters at the White House.

Biden said such an invasion would be met by a "severe and coordinated response, economic response as discussed in details with our allies as laid out very clearly with president Putin".

He added: "But there is no doubt, let there be no doubt at all, that if Putin makes this choice, Russia will pay a heavy price".

Russia has massed tens of thousands of troops on its borders with Ukraine, and Western states fear Moscow is planning a new assault on a country it invaded in 2014. Russia denies this.

Biden said Washington was on the lookout for other scenarios.