President Biden on Thursday said the U.S. has reason to believe that Russia is "engaged in a false flag operation," and said he believes that an invasion into Ukraine "will happen in the next several days."

Tensions have spiked along the line that separates Ukrainian forces from Russia-backed separatists in the country’s east, with the parties accusing each other of intensive shelling.

As he departed the White House, Biden said the threat of a Russian invasion into Ukraine is "very high," and that he believes it could happen "in the next several days."

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"They have not moved their troops out. They've moved more troops in," Biden said. "Every indication we have is they're prepared to go into Ukraine, attack Ukraine."

President Biden walks on the South Lawn of the White House, Thursday, Feb. 17, 2022, in Washington. (AP Photo/Patrick Semansky)

When asked whether diplomacy was still on the table, Biden said yes, pointing to his move to send Secretary of State Antony Blinken to the United Nations Thursday, where he will make a statement and "lay out what that" diplomatic path could be.

Biden said Thursday that despite signs of an imminent invasion, he has "no plans to call Putin right now."

"I guess it will happen," Biden said, referring to an invasion, and adding, again that the "diplomatic path is still available before the deadline."

Defense Secretary Lloyd Austin on Thursday addressed reports of shelling in Ukraine "troubling."

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"Russia continues building up their military presence along Ukraine’s borders including in Crimea, in Belarus and in the Black Sea," Austin said, adding that "in many ways this brings Russian troops right up to NATO’s doorstep." 

"You don’t do these sorts of things for no reason, and you certainly don’t do them if you are getting ready to pack up and head home," he said. 

Earlier this month, the Pentagon announced that it had intelligence that Russia was likely to "fabricate" a pretext for an invasion of Ukraine, which would falsely pin an attack on Ukrainian forces.

Pentagon spokesman John Kirby said that type of plan was "right out of their playbook."

U.S. officials believe that Russia is planning to "stage a fake attack by Ukrainian military or intelligence forces against Russian sovereign territory or against Russian-speaking people to therefore justify their action as part of this fake attack," Kirby explained on Feb. 3.

U.S. Defense Secretary Lloyd Austin attends a news conference following a NATO Defence Ministers meeting in Brussels, Belgium, Feb. 17, 2022. (REUTERS/Johanna Geron)

"We believe that Russia would produce a very graphic propaganda video, which would include corpses and actors that would be depicting mourners, and images of destroyed locations, as well as military equipment, at the hands of Ukraine, or the West," Kirby said, adding that the U.S. has information that the equipment in the video "would be made to look like it was Western supplied" equipment to Ukraine.

"This is just one example," Kirby said. "We’re watching this across the board. We’ve seen these kinds of activity by the Russians in the past." 

And on Thursday, NATO Secretary-General Jens Stoltenberg, when asked by Fox News about the capability of Russia to launch an attack on Ukraine, said Russia has enough troops and resources ready to begin "a full-fledged invasion of Ukraine with very little or no warning time." 

Secretary General Jens Stoltenberg speaks to the press ahead of a meeting of foreign ministers held at NATO headquarters, in Brussels, Belgium, on Nov. 26, 2021. (AP Photo/Olivier Matthys)

"And that is what makes this situation so dangerous," Stoltenberg said. "So we know about their capabilities but of course we don’t know with certainty about their intentions, so it remains to be seen what they will do." 

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"To have so many combat-ready troops in and around Ukraine… it’s not a normal exercise… it’s not a normal activity on their own territory," Stoltenberg continued. "This is something that is threatening an independent sovereign nation and they can launch an attack with hardly any warning time at all and that’s the danger." 

Fox News' Greg Norman, Eddie DeMarche and Ronn Blitzer, and The Associated Press contributed to this report.