Russia's Federal Security Service (FSB) said on Thursday that Britain's Special Boat Service had been operating in Ukraine and helping Ukrainian forces carry out attempted operations against Russian forces.

The Ukraine war has triggered the deepest crisis in Moscow's relations with the West since the 1962 Cuban Missile Crisis. President Vladimir Putin has said that NATO military personnel are present already in Ukraine. The U.S. 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 sabotage soldiers on the Tendrov Split, a sandbar in the Black Sea. It said it had captured a senior Ukrainian naval special forces soldier, and gave his name and date of birth.

The FSB said the Ukrainian special forces unit was "supervised by a unit of the Special Boat Service (SBS) which indicates the direct involvement of Britain in the conflict".

A spokesperson for Britain's Defence Ministry had no immediate response to a Reuters request for comment.

The SBS is a special forces regiment of the British navy that traces its history to the early days of World War Two.

The SBS has served 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.

(Reporting by Guy Faulconbridge; editing by Mark Heinrich)