The British founder of an organisation that trained the Syrian White Helmets emergency response group has died in Istanbul on Monday, sources said.

James Le Mesurier, founder of the Mayday Rescue group, was found dead early on Monday in central Istanbul's Beyoglu district, a Turkish source with knowledge of the case told MEE.

The Turkish security source told MEE that the former British army officer was found dead outside his home on Monday morning.

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“He appeared to have fallen from a height. Current information suggests he had committed suicide,” the source said on condition of anonymity.

A second Turkish source with knowledge of the case told MEE that the building was only accessible by using fingerprint identification.

Preliminary evidence suggests that the system was only accessed by Mesurier and his wife overnight.

"Mesurier and his wife appear to be alone in the building at the time of the death,” the source said.

A security source told Reuters news agency: "Le Mesurier's wife told police that she and her husband had taken sleeping pills around 4am and went to bed.

"She said she was later woken by knocking on the door and discovered that her husband was lying on the street surrounded by police," the source added.

The privately owned Haberturk daily reported that the police, checking the security footage, couldn’t detect anyone other than Le Mesurier and his wife inside the building.

Meanwhile, another source told the privately owned Turkish news-wire Demioren News Agency that police had found his face had been cut by a sharp object.

In an official statement, the Istanbul governor's office said "a comprehensive administrative and criminal investigation on the death of English citizen James Gustaf Edward Le Mesurier" had been opened.

Raed al-Salah, the leader of the White Helmets, confirmed the news to Middle East Eye.

“He was found dead outside of his house this morning,” he told MEE. “The police is investigating it. We don’t have any certain information on what has transpired.”

We have learned with shock and sadness the news of the death of @lemesurierjames, founder and director of the humanitarian organization @MaydayRescue, early on Monday at his home in Tophane in Istanbul, Turkey. Mayday is one of the institutions supporting the White Helmets. pic.twitter.com/czhB3IkdWN — The White Helmets (@SyriaCivilDef) November 11, 2019

Last week Russian foreign ministry spokesperson Maria Zakharova accused Mesurier of being a "former agent" to the United Kingdom MI6 intelligence agency and said he had connections to "terrorist groups".

The White Helmets, known officially as Syria Civil Defence, have been credited with saving thousands of people in rebel-held areas during years of bombing by Syrian government forces and its ally Russia in the country's civil war.

Mayday Rescue is a not-for-profit organisation with offices in Amsterdam and Istanbul whose projects have been funded by the United Nations and various western governments.

Mayday Rescue did not immediately respond to an emailed query by Reuters about Le Mesurier.