The US military said on Tuesday that it could only confirm that about 90 percent of the Syrian city of Raqqa had been retaken from Islamic State militant group, even as US-backed forces declared victory there.

The Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab militias, said the fighting was over and it was clearing Raqqa's stadium of mines and any remaining militants.

A formal declaration of victory in Raqqa, Islamic State group's de facto capital in Syria, will soon be made once the city has been cleared of mines and any possible Islamic State group sleeper cells, said SDF spokesman Talal Selo.

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

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

"We are aware of the reports that ISIS has been defeated in Raqqa," Dillon said, using an acronym for Islamic State group.

"However, clearance operations continue and we expect our Syrian Democratic Force partners to hit pockets of resistance as the final parts of the city is cleared," Dillon said.

He said that in the past few days, about 1,300 civilians had been "assisted to safety" by the SDF and about 350 Islamic State group fighters had surrendered in Raqqa.

Even as the SDF made gains in Raqqa, Dillon said Islamic State group fighters still remained in the middle Euphrates River valley.

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