US-backed fighters have captured 90 percent of the Syrian city of Raqa from the Islamic State group after overrunning five strategic neighbourhoods, a monitor said on Wednesday. 

"Because of the heavy (US-led) coalition air strikes, IS withdrew from at least five key neighbourhoods over the past 48 hours," said Syrian Observatory for Human Rights chief Rami Abdel Rahman. 

"This allowed the Syrian Democratic Forces to control 90 percent of the city."