US-backed forces have seized control of the vast majority of the Syrian city of Raqqa from the Islamic State group after overrunning five strategic districts, a monitor said on Wednesday.

In a statement, the Syrian Democratic Forces said it had opened a new front against IS on the northern edge of Raqqa, describing this as "a feature of the final stages of the Euphrates Wrath campaign, which is nearing its end".

The Euphrates Wrath campaign... is nearing its end - SDF statement

Separately, the Syrian Observatory for Human Rights war monitor said the SDF had taken 90 percent of the city.

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

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

SDF forces walk through a destroyed street in Raqqa (Reuters)

IS has pulled out of the north of the city and abandoned its grain silos and mills.

"IS is now confined to the city centre," Abdel Rahman said.

"After hundreds of their fighters were killed in recent weeks, [those remaining] will not be able to resist much longer in Raqqa as their military equipment and basic necessities are dwindling," he said.

Without food or medical equipment, IS was unable to treat its own wounded and had retreated to the city centre, which it considered "the most secure", he said.

But the battle for the 10 percent of the city still held by IS will likely be tough, as IS had heavily mined the area, Abdel Rahman said.

"The difficulty in advancing and fully clearing these [districts] is linked to the mines that IS has left behind."

IS seized Raqqa in early 2014, transforming the city into the de facto Syrian capital of its self-declared caliphate.

It quickly became synonomous with the group's most gruesome atrocities, including public beheadings.

Backed by US-led coalition air strikes, the SDF spent months encircling the city before entering it in early June.

Hundreds of civilians have been killed in US coalition air strikes. Syrian government shelling and IS mines and snipers, Amnesty International said in a report last month.

Tens of thousands of civilians have fled the fighting in recent months. Estimates of the number still inside the city range from fewer than 10,000 to as many as 25,000.