NATO is greatly ratcheting up its presence near Russia, announcing Monday it will send an additional four battalions to the Baltic states and Poland, in continued tensions with Russia.

NATO Secretary-General Jens Stoltenberg said defense ministers in the alliance will formally approve the plan during meetings that begin Tuesday in Brussels. Approximately 4,000 troops from assorted NATO countries will be involved.

"This will send a clear signal that NATO stands ready to defend any ally," Stoltenberg said.

U.S. NATO Ambassador Douglas Lute said many of the other details will be announced at the July NATO summit in Warsaw, Poland. He said the troops will stay as long as necessary.

The move comes amid the largest war game by Western countries since the Cold War. The game is a 10-day military exercise, featuring 31,000 troops and thousands of vehicles from 24 countries, and has been celebrated by NATO member nations weary of an ever-encroaching Russia.

Russia has periodically held unannounced military exercises on its Baltic border, often in response to NATO moves.

Lithuania's Defense Minister Juozas Olekas would not rule out the threat of imminent Russian aggression and confrontation.

"We cannot exclude it ... They might exercise on the borders and then switch to invasion in hours," he said. The Baltic states also want NATO fighters to protect their skies and are seeking medium-range missile interceptors.

"We need to stop possible air aggression," Olekas said. "We are discussing creating a regional medium-range air defense system together with the Latvians, the Estonians and the Poles."

Other regional officials echoed that sentiment.

"The first and foremost is the defense of our airspace. Air defense is the challenge that needs to solved together with the NATO alliance," said Lt. Gen. Riho Terras, the head of the Estonian defense force.

Lithuanian Foreign Minister Linas Linkevicius agreed.