Finland's border guard is investigating why a Russian aircraft crossed into Finnish airspace on Wednesday, the second such incident this year, Finland's defence ministry said.

A Russian Tu-154 entered half a kilometre into the Finnish airspace at 10.55-10.58 a.m. local time (0855-0858 GMT), southwest of Porvoo on Finland's southern coast, the ministry said in a statement.

"This is being investigated by the border guard. We don't have much detail yet. In general, we have always asked for an explanation (in such incidents)," Defence Ministry spokesman Jyrki Iivonen said.

The Finnish Air Force scrambled a flight to identify the plane but Iivonen said it was unclear if it was a transport or some other kind of plane.

The violation comes after a Russian Ilyushin-76 transport plane infringed Finnish airspace in September in the same area. Russia expressed regret for the violation in September and called it an unintentional incursion.

Air force representatives from Finland, Russia and Estonia met in Helsinki last month to discuss use of the narrow and busy channel of international airspace above the Gulf of Finland.

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