The move comes as tensions have heightened between Moscow and Helsinki over Crimea's reunification with Russia, the Ukraine crisis and Finland's renewed efforts to join NATO.
"The letter reminds them of their responsibilities and what they will be expected to do in the event of a military crisis," the spokesman was quoted as saying by the BBC as saying.
The spokesman meanwhile denied the procedure was in any way related to the current political situation.
The defense ministers of Norway, Sweden, Denmark and Finland, and the foreign minister of Iceland in April published a joint op-ed in the Norwegian Aftenposten newspaper, asserting the need to expand northern military cooperation amid "Russian aggression."
NATO has been strengthening its military presence along Russia's western border since the conflict in southeastern Ukraine escalated. The bloc accuses Moscow of involvement in the fighting, which has been repeatedly refuted by Russia.
Russia has expressed concern over NATO's military buildup along its borders.
