RAQQA (AFP) - US-backed forces combed the ruins of Raqa for survivors and bombs Wednesday (Oct 18), after retaking the Syrian city from Islamic State in Iraq and Syria (ISIS) extremists and dealing their dreams of statehood a fatal blow.

A lightning final assault by the Syrian Democratic Forces on Tuesday saw ISIS defences collapse faster than expected and the SDF claim a landmark victory in the three-year fight against the group.

SDF fighters flushed militant holdouts from Raqqa's main hospital and municipal stadium, wrapping up a more than four-month offensive against what used to be the inner sanctum of ISIS's self-proclaimed "caliphate".

On Wednesday, SDF forces fired into the air and danced the traditional Middle Eastern dabke to music blasting into the otherwise eerie silence of the city.

Inside the stadium, the militia's flag was raised, as bulldozers worked to clear ground of explosives that ISIS has strewn throughout the city.

Many roads were still closed off, and access to the hospital was blocked while fighters worked to clear it.

Teams of SDF fighters were deployed across the rubble-strewn streets to look for unexploded ordnance and booby traps left behind by the militants.

"They are making sure there are no more sleeper cells" in Raqqa, SDF spokesman Mustefa Bali told AFP.

"Mine-clearing operations and the re-opening of the city are under way," Bali said, adding that his organisation would only formally announce the liberation of the city once they are completed.

The SDF and the Kurdish intelligence services issued clear instructions forbidding the tens of thousands of displaced families from attempting to return to their homes.

City unsafe

"We urge our people... who fled ISIS rule not to return to the city for their own security until it is rid of terrorist explosives," the Kurdish internal security services said in a statement.

The loss of Raqqa left ISIS ruling over a rump "caliphate" straddling the Iraqi-Syrian border and covering a fraction of the territory it held when it declared its "state" in July 2014.

The US-led coalition supporting anti-ISIS forces in Iraq and Syria said on Tuesday that the extremists had lost 87 per cent of the territory they had three years ago.

Brett McGurk, the White House's envoy to the multinational coalition, said on social media that IS had lost 6,000 fighters in Raqqa and described the organisation as "pathetic and a lost cause."

Raqqa was one of the most emblematic ISIS bastions, at the heart of both its military operations and its propaganda.

Several of the most high-profile attacks ISIS claimed in the West - such as the 2015 massacres in Paris - are believed to have been at least partly masterminded from Raqqa, earning the city the nickname of "terror central".

Raqqa also featured heavily in the propaganda videos - from public beheadings to trainings - which ISIS used to instill fear among the caliphate's residents and appeal to new recruits globally.

Fate of ISIS fighters unclear

The breakthrough in the months-old operation to retake Raqqa came last week when a local deal was struck for the safe exit of several thousand civilians who had been used as human shields by ISIS and for the surrender of Syrian militants.

It had been believed that up to 400 mostly foreign ISIS fighters remained in the city, prepared for a bloody last stand in their final redoubts.

Yet the sequence that followed the announcement on Sunday of the operation's final phase gives few clues as to their fate.

"Some surrendered, others died," Talal Sello, another SDF spokesman said, without elaborating further or providing figures.

The Syrian Observatory for Human Rights, a Britain-based monitor relying on an extensive network of sources across Syria, said most of the foreign fighters surrendered and were being held by Western intelligence services.

"They are not visible because intelligence services are detaining them," Observatory chief Rami Abdel Rahman told AFP. "French and Belgian jihadists are definitely being held by intelligence."

It was not immediately possible to corroborate his claim.

Colonel Ryan Dillon, the US-led coalition's spokesman, only spoke of four confirmed cases of foreign ISIS fighters surrendering and stressed that they were in SDF custody.

"We as the coalition do not hold or control any of these detainees," he said, adding that the SDF may make separate arrangements with the detained extremists' countries of origin for some of them to be handed over and prosecuted.