Finland has contacted nearly a million reservists by e-mail but an armed forces spokesman on Saturday denied the move was motivated by security concerns.

"This letter to the reservists is to develop better communication with them and not because of the security situation," spokesman Mika Kalliomaa told YLE public television.

Finland said on Tuesday its navy had fired warning shots at a possible submarine off the coast of Helsinki.

The incident came amid an increase in Russian military activity in the Baltic Sea area, including several airspace violations and war planes allegedly flying without their identifying transponders.

This has prompted non-aligned Finland to announce closer military cooperation with its Nordic neighbours -- including NATO members Norway and Denmark.

Finland shares a 1,340-kilometre (830-mile) border with Russia, and has aimed to maintain good relations with its powerful neighbour since the end of World War II.

In October, Sweden's armed forces hunted unsuccessfully for a week for what they believed to be a foreign submarine in waters close to the capital Stockholm, after several observations were made.

Political observers noted that the suspected submarine incursions in Sweden and Finland both occurred shortly after the election of new governments in the two non-NATO countries, and could possibly be interpreted as warnings to the new administrations to steer clear of NATO cooperation.