Russia and Belarus began their largest joint military drills in years Thursday, watched closely by the U.S. and Europe amid tensions over neighboring Ukraine.

Thousands of troops backed by tanks, fighter aircraft and advanced S-400 missile-defense systems are involved in the "Allied Resolve 2022" exercises in Belarus that run to Feb. 20. They include drills near the border with Ukraine as well as close to Poland and Lithuania, both NATO members.

"Russia and Belarus are facing unprecedented threats, the nature and concentration of which is now, unfortunately, much greater and far more dangerous than it was before," Kremlin spokesman Dmitry Peskov said on a conference call Wednesday. While Russia conducts regular drills with Belarus, these exercises "may be on a larger scale than before" in response to pressure from NATO, he said.

Around 30,000 Russian troops may be in Belarus, making it "the largest military buildup there since the Cold War," NATO Secretary General Jens Stoltenberg told reporters Monday. The alliance has moved to reinforce its eastern flank and remains vigilant because "we have seen Russia use military exercises before as a cover for aggressive actions," he said.

Russia has repeatedly denied it plans an attack on Ukraine after the U.S. and its NATO allies warned a buildup of close to 130,000 Russian troops near the Ukrainian border may be preparation for an invasion as soon as this month, including via Belarus from the north. The Kremlin accuses the West of trying to undermine Russia's security by drawing Ukraine closer to the North Atlantic Treaty Organization.

"They've announced that the exercises would last until February 20," French Foreign Minister Jean-Yves Le Drian told France Inter Thursday. "It will be a very significant test — we will see what happens after these exercises, if there is, or not, a process of de-escalation."