Britain's ambassador to the UN on Monday condemned Russian claims that James Le Mesurier, the former British Army officer found dead in Istanbul, was a spy.
Karen Pierce said Mr Le Mesurier, who started Mayday Rescue, which helped start the White Helmets organisation in Syria, was “a true hero” whose efforts had helped civilians in the Middle East.
“I really want to stress that civilian bit in the light of some of the accusations that Russia and others have made,” Ms Pierce said of the former officer's role in helping the White Helmets.
“They have saved countless people from the ravages of [Syrian President Bashar Al] Assad's forces and paramilitaries. He really deserves our respect. We've lost a real humanitarian.”
The British ambassador said she would be following very closely the Turkish investigation into Mr Le Mesurier's death.
He was found dead at his apartment building in Istanbul, reportedly with fractures to his skull and legs.
“I'm sure we'll want to give them any assistance they might require,” Ms Pierce said.
The Russian Foreign Ministry just days ago alleged in a tweet that he was an agent of MI6, Britain's foreign intelligence service.
Ms Pierce spoke of Mr Le Mesurier's service and training at the Royal Military Academy Sandhurst, but said he had died as a civilian.
“I want to take the opportunity to say on the record that the Russian charges against him that came out of the foreign ministry, that he was a spy, are categorically untrue,” she said.
“Above all, he wasn't a serving soldier when he founded Mayday and the White Helmets. The world, and Syria in particular, is poorer for his loss.”
Known officially as Syria Civil Defence, the White Helmets are a voluntary search-and-rescue group formed to respond to bombings by Syrian government forces in opposition-controlled parts of Syria.
They have been credited with saving tens of thousands of lives during Syria's conflict.
A documentary about the group won an Academy Award in 2017.