A US-backed coalition of Kurdish and Arab fighters, says the three-month battle for ISIL's self-declared capital in Syria, Raqqa, has reached its "final stages" with the opening of a new front.

The Syrian Democratic Forces (SDF) said in a statement on Wednesday that it had started a new offensive against the Islamic State of Iraq and the Levant (ISIL) group on the northern edge of Raqqa.

This was "a feature of the final stages of the Euphrates Wrath campaign, which is nearing its end", the statement said.

"We can say that 80 percent of the city of Raqqa has been liberated," it said.

The SDF group spent months encircling the city before entering it in June and sealing off all access routes.

The UK-based Syrian Observatory for Human Rights said on Wednesday that the alliance had taken more than 90 percent of the city, adding that the major remaining hurdle for its forces was a large concentration of mines in the area.

"Because of the heavy coalition air strikes, ISIL withdrew from at least five key neighbourhoods over the past 48 hours," said Rami Abdel Rahman, head of the Observatory.

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

ISIL (also known as ISIS) was now confined to the city centre, in government administrative buildings, the stadium and tunnels.

The remaining ISIL fighters in the city have nearly run out of food and munitions, the Observatory reported.

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SDF launched its offensive on ISIL in Raqqa city in June, backed by US-led air raids and special forces on the ground.

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.

SDF said on Wednesday it had helped hundreds of civilians escape the city in recent days.

Years of conflict

Hundreds of thousands of people have been killed and millions more displaced since civil war broke out in Syria following protests against President Bashar al-Assad in March 2011.

It has since evolved into a complex, multi-front conflict involving government forces, Syrian rebels, Kurdish fighters and armed groups, including ISIL.

ISIL seized Raqqa in early 2014, transforming the city into the de facto Syrian capital of the "caliphate" it declared after taking control of large parts of Syria and Iraq.

It quickly became synonymous with the group's most gruesome atrocities, including public beheadings, and ISIL is thought to have used the city to plan attacks abroad.

In recent months, ISIL has seen the territory under its control diminish quickly.