The U.S.-backed Syrian Democratic Forces (SDF) said Sunday it had launched the operation to liberate Raqqa city from the Islamic State group, also called ISIS. The northern city is the militant group’s headquarters in Syria.

The alliance consists of mostly Arab and Kurdish fighters backed by U.S. air support. Close to 30,000 fighters are taking part in this operation called Euphrates Anger, SDF spokeswoman Jehan Sheikh Amad said.

Amad said the operation, which began late Saturday, would “continue until all objectives are met, namely, seizing and toppling the capital of ISIS.”

“On this occasion, we call on the international community and regional forces to coordinate and take part in the operation to exterminate ISIS,” Amad reportedly said. “We also call on international humanitarian and relief agencies to perform their duties for the people in Raqqa after the city has been liberated.”

ISIS, which took Raqqa from rebel groups in January 2014, aside from using the city as the headquarters for its Syria operations, also runs training camps there. The militant group is under attack from multiple forces with U.S.-backed Iraqi forces attacking the headquarters of its Iraq operations, Mosul.

U.S. Secretary of Defense Ash Carter said in October that operations to liberate Raqqa would “overlap” with the Mosul offensive.

“When it comes to Raqqa, we want a force that ultimately liberates Raqqa that is primarily from the local area. We have trained many of these fighters and that force will continue to grow as we get to the subsequent phases of that campaign,” Brett McGurk, U.S. President Barack Obama’s envoy in the fight against ISIS, said.

“We work closely with the Syrian Democratic Forces. When they are fighting Daesh [ISIS], we do provide air support. So that will continue as they move south against Daesh positions north of Raqqa,” he added.

The SDF consists of the Syrian Kurdish People's Protection Units, which is reportedly the backbone of the alliance. The group has already defeated ISIS in other places, including securing Tal Abyad, a key border town. The Kurdish female Women’s Protection Units, the Syriac Christian militia and Turkmen fighters make up the SDF. They are joined by the Raqqa Falcons Brigade whose troops come from Raqqa.

There are as many as 1,000 armed opposition groups in Syria with about 100,000 fighters, according to the BBC. While all the groups are against Syrian President Bashar Assad’s regime, some, not backed by the U.S.,  are also linked to the al-Qaeda terrorist group.