A friend and former colleague of James Le Mesurier has paid tribute to the British Army veteran, describing him as a "very fine officer."
Karen Pierce, the UK's representative to the United Nations, said the UK is to look closely at an investigation into the circumstances surrounding his death in Turkey
James Le Mesurier was also 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.
Turkish detectives believe he may have fallen to his death from his home in Istanbul.
His friend, Hamish de Bretton-Gordon, a director of the Doctors Under Fire campaign group, has spoken to Forces News about his impression of Mr Le Mesurier.
"We served in the army some time ago," Mr de Bretton-Gordon said.
"He did a relatively short time, I think five or six years.
"Then I met him again in 2014 when he conceived the idea of the White Helmets and set it up - a tremendous achievement, in effect setting up the NHS, the fire brigade and also the ambulance service in war-torn Syria.
"It took a special kind of person to be able to do that.
"He takes tremendous credit, he's had a humanitarian footprint in Syria that very few other people have managed to achieve."
When asked about why the Russians described Mr Le Mesurier as a spy, he said: "James had been under tremendous pressure from Russian and Syrian propagandists and apologists, trying to accuse the White Helmets of being al-Qaeda operatives, and actually a terrorist group fighting against Assad and the Russians which is complete rubbish.
"I saw them in Syria myself and they were first responders, but there was direct targeting of James".
Mr de Bretton-Gordon said the Syrian government and its allies "could well have" regarded James Le Mesurier as an enemy for his work with the White Helmets.
"They were getting increasingly frustrated with the White Helmets posting videos on social media of children being killed and women being blown up in airstrikes by the Russians and by Syrian jets, and they were trying to counter that.
"I know James got a lot of direct vitriol from these people, as do a lot of us who are trying to help on the humanitarian side in Syria, but the White Helmets were the focus of much of the angst and derision from the Russians and the Syrians, and as the head of that organisation, I'm sure he got more of that than most.
"I saw him in person a few weeks ago and we regularly communicated on various bits of social media, and certainly when I saw him in person he was on good form.
"There was nothing to suggest he was under any particular mental pressure, more than you can expect from running a very large organisation in a war zone."