Some Russian military officials are worried about how difficult and costly a full-scale invasion of the Ukraine might be, CNN is reporting.

According to the news organization, the United States has intercepted communications between intelligence and military operatives who are expressing concerns that invading Ukraine may prove to be more difficult and costly than Russian President Vladimir Putin and others at the Kremlin believe.

"In the assessments we see it is clear some people on the [Russian] defense side are not really understanding what the game plan is," the official told CNN. The official added that the assessments suggest the defense personnel think "it's a very difficult game plan to stand up."

Intelligence officials estimate that Russia now has up to 70% of the forces amassed at its border with Ukraine to launch such an invasion, leading President Joe Biden on Monday to warn Americans to leave that country.

"I think it would be wise to leave the country," The Hill reported Biden said, clarifying he was not referring to U.S. diplomatic staff. "I would hate to see them get caught in a cross-fire if in fact [Russia] did invade. And there’s no need for that, and if I had anyone there, I'd say leave."

Biden said that Putin must understand the instability an invasion would bring to the region, and the “heavy price” he would pay on the world stage.

"And I think he has to realize that it would be a gigantic mistake for him to move on Ukraine," Biden said. "The impact on Europe and the rest of the world would be devastating, and he would pay a heavy price." 

A Washington Post report Saturday said an invasion could cause more than 50,000 casualties with the Russian army taking the capital city of Kyiv in “days.”

If that scenario were to unfold, U.S. officials also estimate a humanitarian crisis with up to 5 million refugees trying to escape the country.

Among the 70% of Russian forces currently at the border, U.S. intelligence estimates 83 battalions with 750 troops each, totaling 62,000 of the more than 100,000 believed to be at the border, the Post reported.

"We've long said that Putin continues to add to his options and capabilities, and we've been equally transparent about some of the moves we believe he intends to make to justify some sort of military action," Pentagon press secretary John Kirby told CNN Monday. "We're also going to continue to help Ukraine better defend itself with both lethal and non-lethal assistance."

Putin has said reports of an invasion plan are not true.

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