The incident comes as relations between the neighbouring countries, who share a 1,340-kilometre (830-mile) border, have become increasingly tense after Finland joined NATO last year.

The suspected intrusion into Finnish airspace was believed to have occurred Monday morning near the city of Loviisa, located less than 90 kilometres east from the capital Helsinki on the coast of the Gulf of Finland, the Nordic country's Ministry of Defence said in a statement.

The aircraft remained in Finnish airspace for about two minutes and at its deepest flew 2.5 kilometres into Finnish territory, according to the ministry.

"We take this suspected territorial violation seriously and investigations have begun," Finnish Minister of Defence Antti Hakkanen said in the statement.

Finland's Border Guard service was conducting a preliminary investigation and would provide more information once it was complete, the statement said.

In August 2022, Finland said it believed two Russian fighter jets entered Finland's airspace, also over the Gulf of Finland.

The Nordic nation closed its eastern border in mid-December after nearly 1,000 migrants had arrived without visas at its border crossings with Russia in the previous months.

Helsinki has said the surge in asylum seekers had been orchestrated by Russia, a charge that Moscow denied.