US backed forces in Syria claimed that they had full control of the Islamic State's onetime capital of Raqqa, heralding an end to the militants' presence in their most symbolically important stronghold and raising new questions about the United States' future role in Syria.

Mustafa Abdi, a spokesman for the Syrian Democratic Forces, or SDF, said that military operations had halted and that members of the joint Kurdish-Arab force were clearing the city of explosive devices and hunting for sleeper cells.

The US military said a formal victory announcement will come after SDF forces are sure that no pockets of Islamic State resistance remain in the city, but the SDF portrayed the battle for Raqqa as over.

"There is an air of jubilation in the city," Abdi said. "People are overjoyed that they are finally rid of this scourge."

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Kurdish and Arab fighters took to the streets to celebrate the end of the battle they have fought for four months, climbing onto vehicles and parading around the deserted, destroyed city, according to photographs posted on social media. One showed the offensive's commander, Rojda Felat, waving a large yellow-and-red SDF flag in Naim Square, where the Islamic State carried out its beheadings.

By the time the battle was over, Raqqa had lost all strategic significance to the group that once had used the city to showcase its brutality and plot attacks against the West. The fall of the Iraqi city of Mosul in July and the loss of large areas of territory in eastern Syria to Syrian government forces leave the militants in control of only one sizable stretch of territory, spanning the Iraqi-Syrian border.

Fighters from the U.S.-backed Syrian Democratic Forces (SDF) celebrating their victory in Raqqa, Syria. Photo / AP
Fighters from the U.S.-backed Syrian Democratic Forces (SDF) celebrating their victory in Raqqa, Syria. Photo / AP

But the capture of Raqqa by the SDF, backed by US airstrikes and American advisers on the ground, nonetheless marks a milestone in the US-led war against the Islamic State. In a briefing to reporters in Washington, a US military spokesman, Colonel Ryan Dillon, called it "momentous." He said that the Islamic State has now lost 87 per cent of the territory it once controlled and that 6500 fighters remain, out of tens of thousands at the peak of its prowess.

The victory also intensifies growing questions about what comes next. The remaining Islamic State strongholds in Syria lie to the south in the province of Deir al-Zour, where the Syrian government and its Iranian-backed and Russian allies are making fast progress. The US military will leave it up to the SDF to decide whether it wants to continue to advance into the area, Dillon said.

Perhaps more importantly, the Trump administration has not yet indicated whether it is prepared to stay on in northeastern Syria to provide protection for the fledgling ministate being forged by Syria's Kurds. The experience over the past two days of the Kurds in neighboring Iraq may embolden the Syrian government to challenge the Syrian Kurdish enclave once the Islamic State is vanquished, just as the Iraqi government has moved to dislodge Kurdish forces from the oil-rich city of Kirkuk and other areas they controlled.

Syrian government officials have spoken on several occasions about their determination to regain control over all of the territory they lost to the rebellion against President Bashar Assad, including the area controlled by the Kurds.

"What would be disastrous for Syrian Kurds is a rapid U.S. drawdown in Syria. It would take away their major foreign patron," said Nicholas A. Heras of the Center for a New American Security.

A civilian council comprising Arabs and Kurds is waiting in the wings to take over governance of Raqqa, under the auspices of the Kurdish-led administration running northeastern Syria. But the international community has not committed funds for reconstruction of the devastated city, and the absence of a clear US policy for northeastern Syria risks undermining the gains, cautioned Hassan Hassan of the Washington-based Tahrir Institute.

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Militants of the Islamic State group hold up their weapons. Photo / AP
Militants of the Islamic State group hold up their weapons. Photo / AP

"No one trusts the Americans, not even the Kurds," he said. "To defeat extremism after destroying areas through the necessity of war, you have to deal with the consequences, not just drop bombs and leave because you have an aversion to war."

The offensive to capture the city began in June, with the SDF advancing on the ground as US-led coalition airstrikes pummeled the militants. As was the case in Mosul, victory over the militants has come at a tremendous price. Much of the city now lies in ruins, its residents scattered in displacement camps across the country.

At least 1000 civilians are said to have died, according to estimates by monitoring groups, most of them in the relentless airstrikes. More than 270,000 people fled their homes during the battle, according to the United Nations, and many will find they do not have homes to go back to.

Besieged and severely weakened, dozens of militants had launched a final stand from inside Raqqa's main hospital and stadium. But the end of the battle was hastened by a controversial deal brokered by local officials under which local fighters were offered the chance to escape prosecution if they had not committed crimes, by surrendering to the SDF. Dillon said 350 fighters had surrendered in recent days, including some foreign fighters. They are being screened by the SDF to figure out whether they had participated in killings, he said.

Raqqa was the first provincial capital to fully fall from government control when it was captured in March 2013 by moderate and extremist groups including the al-Qaida-linked Jabhat al-Nusra, then operating as the Syrian wing of the Iraqi-based Islamic State.