Kyiv rejected the accusations and accused Russia of spreading "propaganda".
Russia, which launched its full-scale offensive against Ukraine in 2022, regularly accuses Kyiv and its European allies of brazen sabotage operations on its soil, often without providing evidence.
Russia's Federal Security Service (FSB) said it had "uncovered and thwarted an operation by the... Ukrainian Ministry of Defence and its British supervisors to steal a Russian MiG-31 high-altitude supersonic fighter jet, which carries the Kinzhal hypersonic air-launched missile".
It accused Kyiv of trying to "recruit Russian pilots" by promising them $3 million and citizenship of a Western country.
The FSB said it thwarted a plan by Kyiv to have the plane flown towards a NATO base in the Romanian city of Constanta on the Black Sea, where it would have been "shot down" by air-defences.
State media published an FSB video showing a Russian soldier, face concealed, saying that he had received an email from a Ukrainian intelligence agent trying to recruit him.
Kyiv and Bucharest rejected the accusations.
The "spread of such false accusations is a typical tactic of Russian intelligence services," Ukraine's Center for Countering Disinformation said on social media.
It said Moscow was trying to "intimidate Western societies" and "undermine military assistance to Ukraine".
Romania's foreign ministry spokesperson Andrei Tarnea called it an "invented" story reminiscent of "Soviet spy novels".
"What is real, however, is Russian aggression and Russian provocations, which these stories about planes and spies attempt to cover up," he added.
The accusations had echoes of an incident in August 2023, when a Russian Mi-8 helicopter pilot defected after flying into Ukraine in an operation led by Kyiv's security services.
His crew members were unaware of his intentions and were killed as they tried to escape, both Kyiv and Moscow had said at the time.
The pilot, Maxim Kuzminov, was found dead in Spain in February 2024.
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