Russia's Federal Security Service (FSB) said on Thursday that Britain's Special Naval Service (SBS) had been operating in Ukraine and helping Ukrainian forces carry out operations against Russia, reports "Reuters".
The war in Ukraine has sparked the deepest crisis in Moscow's relations with the West since the Cuban Missile Crisis of 1962. President Vladimir Putin said NATO military personnel were already present in Ukraine. The US and key European allies have said they have no plans to send ground troops to Ukraine.
The FSB, the main successor to the Soviet-era KGB, said it had foiled a plan by British special forces to land Ukrainian soldiers on the Tendrovo Spit, a strip of sand in the Black Sea. It says a high-ranking Ukrainian naval special forces soldier has been captured and gives his name and date of birth.
The FSB said the Ukrainian special forces unit was "monitored by a unit of the Special Shipping Service (SBS), indicating Britain's direct involvement in the conflict".
The SBS is a special forces regiment of the British Navy that traces its history back to the early days of the Second World War.
SBS has been involved in some of the biggest conflicts of the past 70 years, including the Korean War, Northern Ireland, the Falklands War, Sierra Leone, Afghanistan and Iraq.