In this image from video provided by the Turkey-based Kurdish Mezopotamya agency on Oct. 13, civilians flee the last sliver of territory in Raqqa that is still controlled by Islamic State militants. (Uncredited/AP)

The last of the few dozen Islamic State holdouts inside the militant group's de facto capital in Syria were mounting a final stand Sunday, after a stream of militants surrendered under a deal brokered by local officials.

A U.S.-backed alliance known as the Syrian Democratic Forces, or SDF, said that 275 militants had left Raqqa's city center, along with their families, and that they would be interrogated and sent to court if they were suspected to have participated in killings.

The battle for the Islamic State's most famous stronghold began in June, and SDF forces have advanced with the support of heavy airstrikes by the U.S.-led coalition as the city has been turned into a virtual ghost town. The Islamic State has lost all but a sliver of territory in Raqqa. At least 90 percent of the city is believed to be under SDF control, with demining squads combing the streets for explosives laid to deter advancing forces.

[Deal reached to evacuate civilians, local militants from ISIS-held Raqqa]

"There are around 100 foreign fighters still inside a small part of the city," said Mustafa Abadi, a spokesman for the Kurdish-dominated SDF.

Footage broadcast by the ANHA news channel showed the surrendered militants limping out, some of them on crutches, before being herded into a single hall.

The Raqqa Is Being Slaughtered Silently monitoring group, run by former residents of the city, said the detainees had been taken to Hawi al-Hawa prison west of the city, a facility that it described as being under the control of the SDF's intelligence arm.

The U.S.-led international coalition against the Islamic State has distanced itself from the locally brokered surrender. Critics have likened it to a widely derided deal between the Islamic State and the Lebanese Hezbollah militia under which militants and their families were given safe passage from Lebanon to their last stronghold in Syria's eastern border region of Deir al-Zour.

Hundreds of civilians have been evacuated from the city in recent weeks, many of them in pitiful condition after months without adequate food or water. In a video filmed Saturday by the Turkey-based Kurdish Mezopotamya news agency, people can be seen stumbling toward an SDF meeting point, some of them laughing, some crying uncontrollably.

Many face an uncertain future, packed in camps for residents displaced from territories recaptured from the Islamic State.

Mustafa said several of the group's key operators remained inside Raqqa on Sunday night, including Salah al-Fransi, a French jihadist believed to have masterminded attacks on Paris in November 2015 that killed 130 people.

[‘Shocking’ conditions await civilians fleeing ISIS in Syria]

Although Raqqa no longer holds strategic significance for the Islamic State, the loss of its most famous stronghold would deal a deep symbolic blow to a group already on the back foot.

It still holds parts of Deir el-Zour province and Iraq's Anbar province, as well as small, scattered pockets elsewhere. But a self-declared Islamic caliphate spanning Syria and Iraq lies in tatters, with the extremist group reduced to a skeleton force across the two countries.

Islamic State leader Abu Bakr al-Baghdadi remains at large, suspected to be hiding in Syria's eastern borderlands. Those, too, are under pressure from competing U.S.- and Syrian-backed forces racing to rout the group from Syrian territory.

Heba Habib in Stockholm and Zakaria Zakaria in Istanbul contributed to this report.

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