Finland's defense forces have begun informing 900,000 reservists about their duties during war, as military activity mounts in the Baltic Sea region.

The military denied that the letter, originally planned in 2010 to improve contacts with reservists and prepare them for duty, was related to the heightened tensions between the West and Russia amid the Ukraine crisis.

Finland, which shares a 1,300-kilometer (800-mile) border with Russia, has reported several airspace violations by Russian military planes, and NATO says its aircraft conducted more than 400 intercepts last year to protect the airspace of European alliance members — an increase of 50 percent from 2013.

Last week, the Finnish navy dropped depth charges on an underwater intruder in its territorial waters but failed to identify it.