Kurdish-dominated SDF militia declare Raqqa victory against IS, promise a de-centralised Syria

The defeat of the Islamic State group in its Syrian bastion of Raqqa was a "historic" achievement, the US-backed force that led the months-long battle said at an official ceremony in the city on Friday.

"We dedicate this historic victory to all humanity," said Talal Sello, spokesman for the Kurdish-led Syrian Democratic Forces (SDF) that took full control of Raqqa Tuesday.

"We in the general command of the Syrian Democratic Forces announce that we will hand over the administration of the city of Raqqa and the surrounding countryside to the internal security forces in Raqqa," Sello added, referring to part of a civil authority set up for the city.

But Sello said the handover would not be immediate, with SDF fighters still combing the city, which lies in ruins and littered with explosives.

"After the end of clearing operations ... we will hand over the city to the Raqqa Civil Council," he said.

The RCC was created some six months ago, and is made up of local officials who will face the daunting task of rebuilding the city.

The SDF said in a statement on Friday that Raqqa will be part of a decentralised federal Syria, tying the city's future to Kurdish-led plans to set up autonomous regions in the north.

The SDF said the people of the majority Arab city and surrounding province would decide their own future “within the framework of a decentralised, federal democratic Syria”.

“We pledge to protect the frontiers of the province against all external threats,” the SDF said in a statement read out by its spokesman in central Raqqa.

Kurdish-led authorities in other parts of northern Syria are already moving ahead with plans to establish the federal system in areas they control, kicking off a three-phase election process last month in Kurdish majority regions.

The plans for autonomous zones in northern Syria have encountered broad opposition from the United States, neighbouring Turkey, and the Syrian government in Damascus.

Raqqa become the de facto Syrian capital of IS' self-styled "caliphate" in Syria and Iraq after the extremists captured it in 2014.

The SDF, a Kurdish-Arab alliance backed by the US-led coalition, broke into the city in June after months of fighting to surround it.

The Britain-based Syrian Observatory for Human Rights monitor estimates over 3,000 people, at least a third of them civilians, were killed in the fighting.