US-backed forces in northern Syria said Monday paused military operations near a major dam held by ISIS.

A US-backed Kurdish-Arab alliance is advancing against ISIS in the key town of Tabqa near the militant bastion of Raqa in northern Syria, a monitor said Sunday.

The Syrian Democratic Forces now control over half the town of Tabqa, including most of its heart, the Old City, the Syrian Observatory for Human Rights monitor said.

Observatory director Rami Abdel Rahman said fighting was continuing in the town on Sunday morning. The SDF published photos Sunday it said showed items retrieved from newly captured parts of the town, including at least a dozen guns, as well as missiles, ammunition, and an ISIS flag.

The SDF entered Tabqa on Monday as part of their offensive against Raqa, ISIS’s de facto Syrian capital. Supported by US-led coalition air strikes and Special Forces advisers, the SDF surrounded Tabqa in early April.

The town sits on a strategic supply route about 55 kilometers (35 miles) west of Raqa, and served as an important IS command base, housing the group’s main prison.

It is also adjacent to the Tabqa dam, another important strategic prize which remains under ISIS control. The assault on Tabqa began in late March when SDF forces and their US-led coalition allies were airlifted behind ISIS lines.

The city is home to an estimated 85,000 people, including ISIS fighters from other areas. ISIS has put up fierce resistance, including using weaponized drones, a tactic the group perfected in neighboring Iraq.

The group is also fighting street-to-street and using suicide attackers and car bombs to slow the SDF’s advance, according to the Observatory.

The assault on Raqa, dubbed “Wrath of the Euphrates,” was launched in November and has seen SDF fighters capture large swathes of countryside around the city.

More than 320,000 people have been killed in Syria since the country’s war began with anti-government protests in March 2011.