According to Finland's Ministry of Defence, a Russian Tupolev TU-154 airplane is suspected of violating Finland's airspace around 7 am Tuesday over the Gulf of Finland, off the city of Porvoo.
A Defence Ministry spokesman described the time that the plane was in Finnish airspace as "rather brief".
The Tupolev TU-154 is a passenger/cargo plane and the site of the suspected violation is close to a route used by Russian aircraft between St. Petersburg and Kaliningrad.
The Finnish Border Guard is carrying out an investigation into the incident.
The incident came on the eve of the centenary of Finland's declaration of independence from the Russian Empire.
Edit: Story updated at 2.28pm to reflect the fact that the airspace incursion was made by a government plane, not a military aircraft.