A member of the British armed forces died in Ukraine on Tuesday while observing Ukrainian forces test a new defensive capability, Britain's Ministry of Defence said, adding that the accident had occurred away from the front lines.

Britain has been among Ukraine's most outspoken supporters, supplying weapons to Kyiv and training some of its soldiers.

Last month, Britain said it was still making plans as a leading nation in the so-called 'Coalition of the Willing' backing Kyiv for a multinational force in Ukraine after a ceasefire is agreed, despite stiff opposition from Russia.

"Devastated by the death of a UK service personnel in Ukraine," said defence minister John Healey. "My thoughts are with their family, friends and colleagues as they grieve for a loved one. Our hearts go out to them."

Prime Minister Keir Starmer also offered his condolences, saying "their service and sacrifice will never be forgotten".

The British government has never confirmed how many service personnel are in Ukraine but the BBC has reported that a small contingent is supporting Ukrainian forces and is providing security to diplomatic staff. (Reuters)