BEIRUT (AP) — U.S.-backed Syrian fighters launched an operation to retake the last Islamic State-held pocket of the northern city of Raqqa on Sunday after some 275 militants and their family members surrendered.

The Kurdish-led Syrian Democratic Forces said the operation will continue "until all the city is cleansed from terrorists who refused to surrender."

The SDF has been on the offensive in Raqqa since early June and now controls about 90 percent of the city that was once the extremist group's self-styled capital. Most of the fighters who remain in the pocket are foreigners, according to the SDF and opposition activists.

The operation was named after Adnan Abu Amjad, an Arab commander with the SDF who was killed in August while fighting against IS in central Raqqa.

The loss of Raqqa would hand another major blow to IS, which has lost most of the territory it once held in Syria and Iraq. Iraqi forces captured the northern Iraqi city of Mosul — the largest ever held by the extremist group — in July, and Syrian government forces retook the eastern Syrian city of Mayadeen, near the border with Iraq, on Saturday.

IS still holds parts of Syria's Deir el-Zour province and Iraq's Anbar province, as well as small, scattered pockets elsewhere.

On Saturday, the U.S.-led coalition and local officials said Syrian IS fighters and civilians would be allowed to leave Raqqa, but not foreign fighters. The evacuation appeared aimed at sparing the lives of civilians being used as human shields. As of last week, around 4,000 civilians were believed to still be in the city.

The SDF said the initiative by local tribesmen and members of the Raqqa Civil Council "succeed in evacuating civilians who were still in the city and the surrender of 275 local mercenaries and their families." It added that the ongoing offensive aims to "end the presence of mercenaries of the terrorist organization inside the city."

Omar Alloush, a senior member of the Raqqa Civil Council, said the 275 fighters have been detained and are being interrogated to determine who should be referred to legal authorities.

Another Raqqa Civil Council official, Laila Mustafa, said all 275 fighters are Syrians. She added that foreigners were not part of the deal and cannot be "forgiven."

The activist-run Raqqa is Being Slaughtered Silently network said the SDF took the detainees to Hawi al-Hawa prison west of Raqqa. It said the prison is being run by the intelligence arm of the SDF.

Several hundred IS fighters are believed to still be in Raqqa, where many global terror attacks across were initially planned.

Rami Abdurrahman, who heads the Britain-based Syrian Observatory for Human Rights, said among the fighters who remain in Raqqa is an official who planned attacks in France that killed and wounded dozens of people. Abdurrahman said the man is either a French or Belgian citizen of North African origin.

Two Kurdish officials, Nawaf Khalil and Nasser Haj Mansour, said the evacuation took place early Sunday, adding that some gunmen remained in a pocket they still control in Raqqa. Khalil said the civilians who left Raqqa included families of fighters.