Ex-Army officer James Le Mesurier helped to train the White Helmets' volunteers in Syria (Picture: @SyriaCivilDef/Facebook).
The UK will look “very closely” at an investigation into how a former British Army officer died in Istanbul, according to a British diplomat.
James Le Mesurier was the co-founder of the White Helmets civil defence group in Syria and was accused of being a former MI6 agent by Russia last Friday.
Detectives in Turkey believe he may have fallen to his death from his home in Istanbul, according to Turkish media.
Karen Pierce, the UK's representative to the United Nations, called Mr Le Mesurier a "true hero" and "real humanitarian".
She said claims that Mr Le Mesurier was a spy were "categorically untrue".
Last week, Russian Foreign Ministry spokeswoman Maria Zakharova accused Mr Le Mesurier of being a former British agent working in the Balkans and the Middle East.
The Syrian government and its allies, including Russia, have previously been critical of the White Helmets volunteers.
Dame Karen told reporters on Monday: "The causes of death at the moment are unclear.
"We will be looking very closely to see how the investigation goes. I hope the Turkish authorities will be able to investigate thoroughly, and I'm sure we'll want to give them any assistance they might require.
"I do just want to take the opportunity though 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."
Mr Le Mesurier’s body was found near his home in the Beyoglu district by worshippers on their way to a mosque, according to reports.
Turkey's state-run Anadolu news agency said police established that no one had entered or left his home at the time of the incident and believe he may have fallen to his death.
Human rights group Amnesty International has called for an "exhaustive" investigation into his death.
Mr Le Mesurier was the founder and chief executive of May Day Rescue, which founded and trained the White Helmets, also known as the Syria Civil Defence.
He received an OBE for his work in 2016.
The White Helmets group confirmed his death on its Facebook page, and offered "deepest condolences" to Mr Le Mesurier's family.
The group, which has had more than 3,000 volunteers in opposition-held areas, says it has saved thousands of lives since 2013 and documented Syrian government attacks on civilians and other infrastructure.