The incident "occurred over the Gulf of Finland near Helsinki at around 2:00 pm Finnish time (11:00 GMT)" and the border surveillance authority has opened an investigation to confirm the violation, the ministry said in a press release.

Two Sukhoi SU-27 jets "flew in Finnish airspace for about two minutes", according to Niina Hyrsky, the ministry's communications director, who told AFP that Finland had scrambled planes to identify them.

Russian military aircraft regularly cross the Gulf of Finland to link the Russian exclave of Kaliningrad, between Lithuania and Poland.

Finland, part of the Russian empire between 1809 and 1917, fought twice with the USSR during the Second World War -- first as an independent nation and then as an ally of the Germans -- before changing sides in 1944 to fight against the Nazis.

Finland has been neutral or militarily non-aligned since 1955 and is not a member of NATO. It is, however, associated with the Atlantic alliance through various cooperation programmes.