The Portuguese Air Force has said one of its surveillance planes “inadvertently” strayed into Finland’s airspace without permission. The P-3 Orion aircraft based in Poland was on a NATO mission over the Baltic Sea on Monday when it made a turn and unintentionally went about 500 meters into Finnish airspace, according to the statement. The explanation has been sent to Finnish authorities, who had initially reported the violation, AP reports. Portugal is a member of NATO, but Finland is not. The two countries have in the past taken part in joint military exercises.