AIN ISSA, Syria: U.S.-backed forces said Monday they had seized a new neighborhood from Daesh (ISIS) in the militant stronghold of Raqqa in northern Syria.

The Syrian Democratic Forces, an alliance of Kurdish and Arab fighters, have been pressing an operation to capture the extremist stronghold since last year, and entered the city in June.

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

Al-Yarmouk is a large neighborhood on the southwestern outskirts of the city.

"The operation is continuing but there are many fierce clashes," Ahmed said, speaking in the town of Ain Issa, some 50 kilometers 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 said the SDF had advanced in Al-Yarmouk but did not yet fully control the district.

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

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

The monitor estimates the U.S.-backed force currently holds around 35 percent 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 in June.

It has been backed by heavy U.S.-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.