Finland's defense ministry says a Portuguese surveillance aircraft is suspected of having violated the Nordic country's airspace.

Spokeswoman Niina Hyrsky says the aircraft made the alleged airspace violation on Monday at 6 a.m. southwest of the capital Helsinki.

Hyrsky declined Tuesday to comment further or give more details but said the matter is being investigated by the Finnish Border Guard.

Portuguese Air Force spokesman Lt.-Col. Manuel da Costa told the Associated Press he was not aware of the incident but would look into the matter.

Portugal is a member of NATO, but Finland is not.