Russia's Federal Security Service (FSB) said it had foiled a plot by Ukrainian and British spies to tempt Russian pilots to steal a MiG-31 jet armed with a Kinzhal hypersonic missile, state media reported on Tuesday.

The RIA news agency cited the FSB as saying that the hijacked jet was to be flown toward a NATO air base in the Romanian city of Constanta, where it could have been shot down by air defences, the agency reported.

The FSB, the main successor to the Soviet-era KGB, said Ukraine and Britain had planned a large-scale "provocation" using the hijacked aircraft, and that Ukrainian military intelligence had sought to recruit Russian pilots for $US3 million ($A4.6 million) to steal the fighter.

"The measures taken have thwarted the Ukrainian and British intelligence services' plans

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