About 2,000 ISIL fighters are estimated to remain in the Syrian city of Raqqa and "most likely will die" there, Brett McGurk, US special envoy for the coalition against ISIL said.
The assault on Raqqa coincided with the final stages of a campaign to drive Islamic State from the Iraqi city of Mosul, where the Islamist militants were defeated last month.
Brett McGurk said that about 45 percent of Raqqa had been cleared since launching an attack in early June. ISIL has lost 70,000 square km of the territory it once held in the two countries, 78 percent of what they had seized in Iraq and 58 percent of what they held in Syria.