The British founder of an organisation that trained the 'White Helmets' has been found dead in Istanbul. 

James Le Mesurier, an ex-British Army officer and founder of the Mayday Rescue group, was found early on Monday near his home in central Istanbul's Beyoglu district, according to Reuters. 

The Istanbul Governor's Office said an investigation had been launched.

Reuters news agency says an unnamed security source told it  Le Mesurier was believed to have  fallen from the balcony of his home office and his death was being treated as suspected suicide.

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

Mayday Rescue, a not-for-profit organisation, began its operations in 2014 and established an office in Istanbul in 2015 to support its Syria project. According to the group's website, Its work has been funded by the United Nations and the governments of various nations, including Germany, the UK and US.

The White Helmets, known officially as the Syria Civil Defense, divide opinion depending on which side of the Syrian War a nation is on. The US and Britain, among other western countries, have hailed the volunteer force, crediting them with saving thousands of lives with their response to airstrikes from the Syrian government and their Russian allies. 

Both Moscow and Damascus accuse the group of being tools of Western propaganda and linked to jihadist terrorist organisations. 

The Russian Foreign Ministry said on Twitter on Friday that the White Helmets help "the most dangerous terrorist groups," and that Le Mesurier was a former British agent with reported "connections to terrorist groups."

Syria Civil Defense posted on Twitter, expressing "our deepest sorrow and solidarity" with Le Mesurier's family. "We also must commend his humanitarian efforts which Syrians will always remember," it read.

A statement on Mayday Rescue's website, eulogized Le Mesurier. "James dedicated his life to helping civilians respond to emergencies in conflicts and natural disasters. 

"Nowhere was the impact of his important work felt so strongly as in Syria, where Mayday supports a network of volunteer rescue workers known as Syria Civil Defense, also known as The White Helmets. Their work has saved countless lives of civilians affected by the conflict."