Russia-Ukraine updates: US to ban Russian carriers from its airspace

President Joe Biden told ABC News Chief White House Correspondent Cecilia Vega on Thursday morning that he believes Russian President Vladimir Putin will go through with an invasion of Ukraine within days.

Asked when departing the White House if it's his sense that an invasion would happen, Biden said, "My sense is this will happen in the next several days."

The president accused Russia of engaging in a "false flag operation" and said that despite the claims of pulling troops back, the Kremlin has actually moved more troops closer to Ukraine's border.

"They have not moved any of their troops out. They have moved more troops in," he said. "We have reason to believe that they are engaged in a false flag operation, to have an excuse to go in. Every indication we have is they're prepared to go into Ukraine and attack Ukraine."

Although he said he has not yet read Putin's written response to the U.S. and that he has no plans for a call with the Russian leader, Biden repeated that there is still a path open to diplomacy.

Russia insisted again Thursday that some troops massed near Ukraine are returning to base, far from the border.

Russian Ministry of Defense spokesman Igor Konashenkov said in a statement that units from the Western and Southern Military Districts, which were stationed in western Russia and Crimea, are now moving back to their permanent bases. He said Russian troops taking part in drills in Belarus will also return to base once they finish the exercises this weekend.

Konashenkov noted that the first units have already arrived in Nizhny Novgorod, Chechnya and Dagestan, hundreds of miles from the border with Ukraine.

-ABC News' Patrick Reevell

The United States has received a response from Russia regarding the discussion over Moscow's demands for security guarantees that NATO not expand, a senior official with the U.S. Department of State said Thursday.

"We can confirm that we have received a response from the Russian Federation," the official said. "It was delivered to Ambassador Sullivan in Moscow today."

The U.S. government, at Russia's insistence, sent written responses to two draft treaties Moscow published demanding guarantees that Ukraine will never join NATO and that the military alliance pull back its infrastructure from Eastern European countries that joined after the Cold War. Washington rejected those guarantees as non-starters but offered to discuss some confidence-building measures.

Russian Foreign Minister Sergey Lavrov said earlier Thursday that Moscow was planning to send its formal response to the Washington soon.

-ABC News' Conor Finnegan

U.S. Ambassador to the United Nations Linda Thomas-Greenfield warned Thursday that Russia appears to be readying for an "imminent invasion" of neighboring Ukraine.

"The evidence on the ground is that Russia is moving toward an imminent invasion," Thomas-Greenfield told reporters during a background briefing call. "This is a crucial moment."

"Our goal is to convey the gravity of the situation," she added. "That’s why Secretary Blinken is coming to New York to signal our intense commitment to diplomacy, to offer and emphasize the path toward de-escalation and to make it clear to the world that we are doing everything -- everything -– we can to prevent a war."

Blinken has already boarded his flight to Munich but will be flying to New York City first to address the U.N. Security Council session at 10 a.m. ET on Thursday.

-ABC News' Conor Finnegan