The British founder of an organization that trained the Syrian White Helmets emergency response group has died in Istanbul, three people with knowledge of his death said on Monday.

James Le Mesurier, founder of the Mayday Rescue group, was discovered dead early on Monday near his home in central Istanbul's Beyoglu district in Turkey, a neighbour said.

A security source said it was believed Le Mesurier had fallen from the balcony of his home and his death was being treated as a suspected suicide. A third person, a diplomat, said the circumstances of his death were unclear.

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 and Russian forces in the country's more than eight-year civil war.

White Helmets members say they are neutral. Syrian President Bashar al-Assad and his backers describe them as tools of Western propaganda and Islamist-led insurgents.

Turkish police officers stand outside the home of Le Mesurier in Istanbul on Monday. (Kemal Aslan/Reuters)

Mayday Rescue, a non-profit organization, began its operations in 2014 and established an office in Istanbul in 2015 to support its Syria project. Its projects have been funded by the United Nations and various governments, its website said.

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

The death of James Le Mesurier is a terrible loss. He had an enormous impact on the world through his work with the courageous White Helmets volunteers. —@cafreeland

A former British army officer, Le Mesurier was appointed to the Order of the British Empire (OBE) by Queen Elizabeth in 2016 for services to Syria Civil Defence and the protection of civilians in Syria.

The security source said Le Mesurier's wife told police she and her husband had taken sleeping pills around 4 a.m. and went to bed. She said she was later awoken by knocking on the door and discovered her husband was lying on the street surrounded by police, the source added.