Making a chilling and high-profile entry onto the Syrian battlefield in May last year with a public execution in Raqqah's main square, the group, formerly called al-Qaida in Iraq, quickly extended its sphere of influence amid the chaos of Syria's rebel-held regions.
But ISIS, estimated to number around 6,000 fighters, has spread itself thin. The group's fighters were pushed from a swath of villages and towns in Aleppo and Idlib provinces last week.
Rebels claimed to have expelled ISIS from Raqqah's post office and another of its bases on Monday, freeing prisoners.
"It was like Guantanamo," said one man in a video posted online Monday. He was part of a group of at least 20 prisoners who were reportedly held by ISIS. The group has built a reputation for kidnappings and intimidation, leading to an outburst of anger in demonstrations against ISIS last week.
The battles against the extremists are being fought by a tangle of groups with competing local interests. The offensive has been led by the Syrian Revolutionary Front and the newly formed Jaish al-Muhajareen - both affiliated with the Western-backed Supreme Military Council, though the body lacks command and control. They have been joined by fighters from the Islamic Front, a powerful grouping of Islamist rebel units that was formed late last year.
Lt. Col. Mohammad al-Abboud, the Supreme Military Council's representative for the eastern front, which includes Raqqah, said he expected it to take at least a week to drive al-Qaida-linked militants from the city. He said at least 20 rebels had died since fighting broke out in Raqqah on Sunday night, though a number of bodies were impossible to retrieve due to sniper fire. He estimated that about 70 ISIS members were killed. Videos showed black-clad militants lying dead in the streets.
The timing gives rebels a chance to present themselves as a counterweight to al-Qaida at the Syrian peace conference, scheduled for Jan. 22 in Geneva.
"The direct reason why we started the fight is because the acts of the Islamic State weren't bearable anymore," said Abboud. "But we can't deny that there are strategic reasons. One is to show we are a national army that is not conservative and extremist, and to show we are supporting our people."
Still, with the Supreme Military Council little more than a "figurehead," said Salman Shaikh, director of the Brookings Doha Center, the challenge will be getting an opposition delegation to the table that represents fighting groups on the ground.
"Geneva is on the road to ruin," he said. "We are looking at a landscape whereby people on the ground have moved on, though the international effort continues."
The United Nations said it sent out invitations to the talks on Monday. But State Department officials said they see little chance of including Iran as an official participant; one said it was also "less likely than likely" that Iran could play even a sideline role. The person requested anonymity on grounds that the United Nations, not the United States, is the conference host.
Iran nevertheless could do several things to show good faith, the officials said, such as urging the Syrian government - Tehran's ally and military client - to stop the bombardment of civilians in Aleppo and elsewhere and encouraging better access for humanitarian organizations in conflict areas.
Those steps could help "predispose" opponents of any Iranian participation to change their minds, one official said.
Iran would not have to announce its intercession but could play a helpful role with Syria behind the scenes, one official said, adding: "Public or private, we'd take it either way."
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Washington Post correspondent Ahmed Ramadan in Beirut and staff writer Anne Gearan in Brussels contributed to this report.