The Syrian Democratic Forces have spent months closing in on the IS bastion and entered the city’s east and west for the first time last month.

U.S.-backed forces in Syria have breached the wall surrounding the Old City of Raqqa as they try to retake the city from the Islamic State group, the US Central Command has said.

“Coalition forces supported the SDF advance into the most heavily fortified portion of Raqqa by opening two small gaps in the Rafiqah Wall that surrounds the Old City,” Centcom said in a statement, referring to the Syrian Democratic Forces.

U.S.-backed fighters pierced jihadist-held Raqqa from the south for the first time on Sunday, crossing the Euphrates River to enter a new part of the Syrian city, a monitor said.

The Syrian Democratic Forces have spent months closing in on the IS bastion and entered the city’s east and west for the first time last month.

Centcom said the SDF faced heavy resistance at the wall from IS fighters who used it as a combat position and planted mines and improvised explosive devices against advancing U.S.- backed soldiers.

“Conducting targeted strikes on two small portions of the wall allowed coalition and partner forces to breach the Old City at a locations of their choosing, denied ISIS the ability to use pre-positioned mines, IED and VBIEDs, protected SDF and civilian lives, and preserved the integrity of the greatest portion of the wall,” the US statement said.

“The portions targeted were 25-metre sections and will help preserve the remainder of the overall 2,500-metre wall,” it added.

According to the coalition, an estimated 2,500 IS jihadists are defending the northern city.

IS overran Raqqa in 2014, transforming it into the de facto Syrian capital of its self-declared “caliphate,” which it declared three years ago.

The city became infamous as the scene of some of the group’s worst atrocities, including public beheadings, and is thought to have been a hub for planning attacks overseas.

The United Nations has expressed concern for up to 100,000 civilians it says are still trapped in the city.