Updated February 17, 2022 at 9:23 PM ET

President Biden says the U.S. believes Russia is continuing to move more troops to the border with Ukraine and that the chances it will invade its neighbor are "very high."

"My sense is that it will happen in the next several days," Biden told reporters outside the White House Thursday as he left on a trip to Ohio.

"They have not moved any of their troops out. They've moved more troops in," Biden responded when asked why he believes the threat was very high. "We have reason to believe they are engaged in a false flag operation," which would provide "an excuse to go in," he said.

Russia has claimed in recent days that it has pulled troops away from Ukraine, but the Biden administration has said it has confirmed that there's been an increased troop presence at the border by as many as 7,000 troops.

"Every indication we have is they're prepared to go into Ukraine, attack Ukraine," Biden said.

He said the U.S. embassy in Moscow had received a letter from Russian President Vladimir Putin who was responding to U.S. proposals on security measures but that he had not yet read it. He said he had no plans to call Putin.

Meanwhile, Russia has expelled the second-ranking U.S. diplomat in Moscow.

U.S. Deputy Chief of Mission Bartle Gorman was expelled as part of an ongoing tit-for-tat of diplomatic expulsions between the U.S. and Russia, a U.S. embassy official tells NPR's Charles Maynes. The U.S. is calling the expulsion "escalatory" and "unprovoked."

A White House official said in a statement that Biden "will speak with Transatlantic leaders on a phone call [Friday] afternoon about Russia's buildup of military troops on the border of Ukraine and our continued efforts to pursue deterrence and diplomacy."

Copyright 2022 NPR. To see more, visit https://www.npr.org.