(The Hill) – NATO Secretary-General Jens Stoltenberg on Sunday said “Russia is preparing for an invasion of Ukraine,” pointing to Moscow’s troop buildup on the Ukrainian border and the continuation of military exercises in the region.

“Last week, actually, Russia said that they will step back — Russia’s stepping up with more troops and higher and even closer to the to the Ukrainian border,” Stoltenberg said during an appearance on CBS’s “Face the Nation.”

“And also the fact that this exercise they said should end today will now continue. So all of this fits into the picture that Russia is preparing for an invasion of Ukraine,” he added, referring to joint Russian drills with Belarus.

Asked by moderator Margaret Brennan if Russian President Vladimir Putin overseeing some strategic nuclear exercises over the weekend was an attempt to intimidate NATO, Stoltenberg said “it is a way for them to demonstrate all the capabilities they have.”

“This is a regular exercise but it was planned for last fall, and now it happens in the midst of this significant Russian military buildup in and around Ukraine with the largest concentration of the combat troops we’ve seen in Europe since the end of the Cold War,” he said.

Stoltenberg’s comments come after President Biden on Friday said he is “convinced” that Putin has “made the decision” to invade Ukraine. Biden did, however, emphasize that a diplomatic path forward remains open if Putin decides to de-escalate.

One of Russia’s demands amid the increased tensions is that the U.S. and NATO prohibit Ukraine from joining the military alliance. The U.S. and allied nations, however, have said such a request is a nonstarter.

Stoltenberg said on Sunday that a timeline of Ukraine joining NATO does not exist, emphasizing that the member nations will determine the path forward.

“We’re not set any timeline for that, but NATO allies have been very clear that it is for the Ukraine and 30 NATO allies to decide on membership, not Russia,” he said.