Witnesses say US-backed militias have raised a flag inside Raqqa stadium, as a four-month battle to take Islamic State's Syrian capital came to an end.

Key points: US-led forces have been fighting IS in Raqqa since June

US-led forces have been fighting IS in Raqqa since June Fleeing Islamists leave behind IEDs and booby traps

Fleeing Islamists leave behind IEDs and booby traps Stadium and hospital became the last major positions held by IS

The fighting was over but the alliance of Kurdish and Arab militias was clearing the stadium of mines and any remaining hiding militants, said Rojda Felat, commander of the Raqqa campaign for the Syrian Democratic Forces (SDF).

"Now we are witnessing the liberation of the city of Raqqa," SDF spokesman Jihan Ahmed said.

"After we clear the areas, thanks to all our sacrifices, we will rebuild Raqqa so that it will be more beautiful than it was before."

The US military said it could only confirm about 90 per cent of Raqqa had been retaken from militants, despite the victory declaration from the SDF.

Colonel Ryan Dillon, a spokesman for the US-led coalition, said about 100 fighters still remained in the city and he expected the SDF to face resistance from remnants of the militant group.

"We are aware of the reports that ISIS has been defeated in Raqqa," he said.

"Clearance operations continue and we expect our Syrian Democratic Force partners to hit pockets of resistance."

The Syrian Observatory for Human Rights, a local war monitor, said IS was completely cleared from the city.

The fall of Raqqa, where IS staged euphoric parades after its string of lightning victories in 2014, is also a potent symbol of the jihadist movement's collapsing fortunes.

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The SDF, backed by a US-led international alliance, has been fighting IS inside Raqqa since June.

Another witness said militia fighters celebrated in the streets, chanting slogans from their vehicles.

A group of militia fighters and commanders clasped their arms round each other, smiling, in a battle-scarred landscape of rubble and ruined buildings at a public square.

The flag in the stadium and others being waved in the city streets bore the yellow background and red emblem of the Kurdish YPG, the strongest militia in the SDF.

Islamic State has lost swathes of territory in Syria and Iraq this year, including its most prized possession, Mosul, and in Syria it has been forced back into a strip of the Euphrates valley and surrounding desert.

Jihadists leave burnt weapons, documents

Sorry, this video has expired A new video released from Raqqa shows scores of civilians fleeing areas held by the Islamic State group.

The SDF took the hospital in fierce fighting overnight and early on Tuesday, said spokesman Mostafa Bali in a statement.

"During these clashes, the national hospital was liberated and cleared from the Daesh mercenaries, and 22 of these foreign mercenaries were killed there," said Mr Bali, using the Arabic acronym for IS.

An SDF field commander who gave his name as Ager Ozalp said three militiamen had been killed on Monday by mines that had become an IS trademark in its urban battles.

Another field commander, who gave his name as Abjalal-Syriani, said SDF fighters had found burned weapons and documents in the stadium.

The stadium and hospital became the last major positions held by IS after the departure of some of its fighters on Sunday, leaving only foreign jihadists to mount a last stand.

The convoy of IS fighters that evacuated the city took civilians with them as human shields, a militia spokesman said.

The SDF has been supported by a US-led international coalition with air strikes and special forces on the ground since it started the battle for Raqqa city in early June.

The final SDF assault began on Sunday after a group of Syrian jihadists quit the city under a deal with tribal elders, leaving only a core of up to 300 fighters to defend the last positions.

IS militants have been holed up in increasingly smaller areas of the city. ( Reuters: Erik De Castro )

Raqqa was the first big city IS captured in early 2014, before its rapid series of victories in Iraq and Syria brought millions of people under the rule of its self-declared caliphate, which passed laws and issued passports and money.

It used the city as a planning and operations centre for its warfare in the Middle East and its string of attacks overseas, and for a time imprisoned Western hostages there before killing them in slickly produced films distributed online.

The SDF advance since Sunday also brought it control over a central city roundabout, where IS once displayed the severed heads of its enemies, and which became one of its last lines of defence as the battle progressed.

AP/Reuters