Finland has sent letters to nearly a million reservists to collect their contact details and remind them of their duties in case of combat, a defence ministry spokesman said.
The letter tells reservists which regiment or unit to join in the event of war, he told the BBC.
He insisted that the correspondence was not related to Russia's annexation of Crimea or recent fighting in Ukraine.
However, neutral Finland has increased co-operation with Nato this year.
In April the Finnish navy dropped depth charges in waters near Helsinki as a warning to a suspected submarine, which some media reports said was Russian.
The air force in recent months has also had to deal with some airspace violations by Russian warplanes.
Russia has repeatedly warned Finland not to join Nato and has criticised its co-operation with Nato members.
Finland's defence ministry said letters to conscripts were sent throughout May to inform them of changes to the structure of Finland's military.
"The letter reminds them of their responsibilities and what they will be expected to do in the event of a military crisis," the spokesman told the BBC. "The process was started before events in Crimea and Ukraine and is done in part to ensure that we have the right contact details."
But recipients say it is the first time such correspondence has been sent for many years and that the mass communication tactic reflects the concern of the authorities about Russia's intentions.
The Soviet Union invaded Finland in 1939, seizing more than 10% of the country's territory before a peace deal was signed in 1940.
Finland was part of the Russian empire for more than 100 years before it won independence in 1917.
During the Cold War, Finland was officially neutral, but remained under the influence of its neighbour. It forged close ties with the Soviet Union.
Finland shares a 1,340km (833-mile) border with Russia. It has a system of universal male conscription under which all men above 18 serve for 165, 255 or 347 days.
The regular army has about 12,000 soldiers and can rapidly expand to about 280,000 troops if reservists are called up, the defence ministry says.