Syrian Democratic Forces (SDF) said Monday they had seized the Al-Yarmuk district from the militant Islamic State group in the jihadist stronghold of Raqa in northern Syria.

Al-Yarmuk is a large neighbourhood on the southwestern outskirts of the city.

The SDF, an alliance of Kurdish and Arab fighters backed by the US, have been pressing an operation to capture the militant stronghold since last year, and entered the city in June 2017.

“The Al-Yarmuk district was liberated yesterday,” the SDF's spokeswoman for the Raqa operation Jihan Sheikh Ahmed told AFP.

“The operation is continuing but there are many fierce clashes,” Ahmed said, speaking in the town of Ain Issa, some 50 kilometres north of Raqqa.

“We are taking steady and sound steps. What is important to us is not speed, but liberating civilians and eliminating Daesh,” she added.

The Britain-based Syrian Observatory for Human Rights monitor said the SDF had advanced in Al-Yarmuk but had not gained full control the district yet.

The monitor said the militia held the western portion of the district but that heavy fighting was continuing in the area.

It also reported that hundreds of civilians have fled IS-held parts of the city towards areas that are now controlled by the SDF in the last 48 hours.

The monitor estimates the US-backed force currently holds around 35 per cent of the city.

The SDF began an operation to capture Raqqa in November 2016 and spent months taking territory around the city before finally entering it last month.

It has been backed by heavy US-led coalition air strikes, including on Monday, which killed at least three civilians, according to the Observatory.

More than 330,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011.