Finland's military said Thursday it was planning a quicker call-up system for reservists due to the Ukraine crisis and the changing security situation around the country, which borders Russia.

The military has begun asking reservists "if they are ready to join our troops quicker than required by law, which is three months," Colonel Mika Kalliomaa of the Finnish Defence Forces told AFP.

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

But the Nordic country has observed an increase in Russian military activity in the Baltic Sea area, including several airspace violations and war planes allegedly flying without their identifying transponders.

Asked if the quicker call-up system was linked to the Ukraine crisis, Kalliomaa said: "The security environment around Finland and in Europe has changed in the last year."

"All these changes ... started from this crisis," he said.

The quicker call-up system is "one thing we are doing to ensure that our readiness is quick enough," he added.

He would not disclose any details about which troops were concerned or their tasks.

In late April, the Finnish navy fired warning shots at a possible submarine violating its waters off the coast of Helsinki.

The suspected sub was widely believed to be Russian, though the military said it was never able to determine whether a foreign power had in fact entered its waters.

Finland said in May that it had sent letters to nearly a million reservists but denied that move was motivated by security concerns, saying it was aimed at "better communication".