A Sukhoi SU-30 fighter and an IL-78 tanker from the Russian exclave of Kaliningrad passed through Lithuanian airspace for 700 metres (2,300 feet) before leaving it after 18 seconds, probably during aerial refuelling training, according to a short statement from the NATO member country's military.

Later Thursday, the Lithuanian foreign ministry said it had summoned the charge d'affaires from the Russian embassy and issued a "strong protest".

Vilnius urged Russia to "immediately explain" the reasons for the violation of Lithuanian airspace and to take "all necessary measures to prevent such incidents from happening again in the future", the statement said.

Russia's defence ministry later denied the incursion had taken place.

"The flights were conducted in strict compliance with the rules for using airspace above Russian territory. The aircraft did not deviate from their route and did not violate the borders of other states," it said on Telegram.

Two Spanish airforce Typhoon Eurofighters -- part of a NATO air patrol mission in the Baltic -- were quickly deployed after the planes breached Lithuania's border.

The three Baltic countries, all NATO members and firm supporters of Ukraine, border either Russia or its ally Belarus and have been exposed to violations of their territory by Russian planes or drones.

In September, three Russian MiG-31 fighter jets entered Estonian airspace over the Gulf of Finland and remained there for approximately 12 minutes.

That incident prompted Tallinn to request an extraordinary meeting of the UN Security Council and the activation of Article 4 of the Atlantic Treaty, which provides for consultations between allies in the event of a threat to one of its members.