The Eurofighter Typhoons launched early Monday from their base in Romania after the Russian Su-24 Fencer planes were spotted, the RAF said in a statement.

It said there had been "significant Russian air activity through most of the night".

The Russian aircraft eventually turned towards Crimea, the RAF said.

The operation "was in accordance with the NATO Enhanced Air Policing (eAP) mission, whereby the RAF operates alongside its NATO ally to deter Russian aggression, reassure our friends in Romania and assure NATO allies of our commitment to collective defence," the statement said.

Romanian Defence Minister Mihai Fifor earlier said such "provocations" by Russia had become "frequent" in recent months.

Four RAF Typhoons have been deployed at the Mihail Kogalniceanu air base in southeast Romania where several hundred US troops are also stationed.

The move was part of NATO's beefed-up defences on its eastern flank following Moscow's annexation of Crimea in 2014.

The US Air Force has four F-15C Eagles based in neighbouring Bulgaria.