James Le Mesurier, founder of the Mayday Rescue group, was found dead early on Monday near his home in central Istanbul's Beyoglu district, the neighbor said. The diplomat said the circumstances around 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 civil war.
Mayday Rescue is a not-for-profit organization with offices in Amsterdam and Istanbul whose projects have been funded by the United Nations and various governments. Mayday Rescue did not immediately respond to an emailed query about Le Mesurier.