NATO jets scrambled to intercept Russian military aircraft as they neared Latvian airspace, officials said on Wednesday.

Estonian radar detected the aircraft over the Baltic Sea on Tuesday night, NATO said. Other than the lead aircraft, NATO said, none of the other Russian military aircraft was on a flight plan.

NATO sent jets to identify the planes and later reported that the military aircraft flew on into Russian airspace.

NATO didn’t say how many Russian aircraft were involved. The flights come as Russia's Northern Fleet has been placed on full combat alert for military exercises involving nearly 40,000 troops and 50 warships.

The exercises have rattled nerves in nearby NATO states, including Latvia, where U.S. troops and equipment recently arrived for NATO training, and where fears are growing about Russian President Vladimir Putin's next move.

At the same time on Wednesday, Putin joined a celebration in Moscow's Red Square, where Russians celebrated the one-year anniversary of the annexation of Crimea.

NATO has condemned the annexation as an illegal territory grab and is boosting its troop presence in the region in what officials say is an effort to discourage Putin from encroaching into other countries.

Putin describes the annexation as a “reunification,” saying that Crimea’s residents overwhelmingly voted to be part of Russia. (CNN)