US-backed forces took full control of Raqa from the Islamic State group on Tuesday, defeating the last jihadist holdouts in the de facto Syrian capital of their now-shattered "caliphate".

The victory caps a battle of more than four months for Raqa, and hammers another nail in the coffin of the jihadist group's experiment in statehood, which has collapsed in the face of offensives in Syria and Iraq.

Inside Raqa, joyous fighters from the Kurdish-Arab Syrian Democratic Forces celebrated and raised their yellow flag in the city's Al-Naim traffic circle, which became known as "Hell Roundabout" after it was used for gruesome public executions.

"Hell Roundabout is now Al-Naim Roundabout again," the fighters cheered, surrounded by crushed buildings and charred cars damaged in the ferocious battle for the city.

The SDF broke into Raqa in June, after months of fighting to surround the city, and on Tuesday flushed the last few hundred IS fighters from their remaining positions in the main hospital and the municipal stadium.

"Everything is finished in Raqa, our forces have taken full control of Raqa," the alliance's spokesman Talal Sello told AFP.

He said the SDF was combing the city for any remaining jihadists who had not surrendered or been killed.

"The military operations in Raqa have finished, but there are clearing operations now under way to uncover any sleeper cells there might be and remove mines," he said.

- More than 3,000 dead -

The announcement came just days after the SDF said it was launching the final phase of its operation to retake the city.

There had been fears that the force, backed by the US-led coalition battling IS in Syria and Iraq, could get bogged down in a protracted battle for the last 10 percent of the city where the jihadists had prepared for a final stand.

But on Tuesday they captured the hospital and stadium in quick succession, effectively ending IS's more than three-year presence in the city.

Sello said an official statement announcing "the liberation of the city" would be made soon.

The breakthrough in the operation, which was launched on June 6, came after a deal was struck allowing the evacuation in recent days of civilians who had been held as human shields.

Under the deal, a total of 275 Syrian IS fighters and relatives also surrendered to the SDF, though it was unclear whether they would be given safe passage elsewhere.

The battle for the city was fierce, with the Syrian Observatory for Human Rights monitor saying Tuesday more than 1,000 civilians had been killed in the fighting.

The Britain-based group put the overall death toll for the battle at 3,250, with 1,130 civilians among them, but said hundreds more were still missing.

Tens of thousands of civilians fled the fighting, some leaving ahead of the SDF's arrival, and others escaping towards the militia as they advanced in the city.

- IS reduced to 'dwarf territory' -

For Umm Abdullah, a Raqa native who fled the city three years ago, news of its capture from IS was overwhelming.

"I can't describe my happiness," the 44-year-old told AFP in the town of Kobane, 100 kilometres (70 miles) north of Raqa.

"When my sister told me it had been freed, she started to cry, and then I started to cry. Thank God, thank God."

After IS captured Raqa in 2014, the city become synonymous with the jihadist group's worst abuses and was transformed into a planning centre for attacks abroad.

Its capture leaves the group with little more than a "dwarf territory" in neighbouring Deir Ezzor province, said Nicholas Heras, a fellow at the Centre for a New American Security think tank.

"IS will be mainly boxed into a strip of territory running along the Middle Euphrates River Valley in the province of Deir Ezzor," he told AFP.

"This will be the centre of gravity for IS in Syria."

The jihadists face two separate assaults in the province, which neighbours Iraq, including a campaign by the SDF.

A Russian-backed Syrian regime campaign has separately retaken swathes of territory in the province, further reducing a "caliphate" that three years ago was roughly the size of Britain.

The Observatory said regime forces had brought the entire area between Deir Ezzor city and Mayadeen, which was retaken on Saturday, under their control following a major military offensive.

"These are not desert areas, they are villages along the Euphrates that were IS strongholds," the monitor said.

"The Islamic State group is collapsing under pressure from the regime in Deir Ezzor province," it said.

IS also controls territory in neighbouring regions on the Iraqi side of the border, where the jihadists are facing another US-backed offensive by Iraqi pro-government forces.