Some 2,000 ISIS terrorists remain in the Syrian city of Raqqa amid an offensive to recapture the group's stronghold, the BBC quoted a senior US official as saying Saturday.
Brett McGurk, special envoy for the coalition against ISIS, says US-backed forces have seized about 45% of Raqqa since the operation started in June.
He said that the group is fighting for its own survival and that the terrorists are likely to die in the city.
ISIS seized Raqqa in 2014, proclaiming it the capital of a "caliphate".
The Syrian Democratic Forces (SDF) have been gradually advancing on the city since November, and launched an offensive to take it on 6 June.
It is unclear how many civilians are still in Raqqa, but the UN estimates this number to be between 20,000 and 50,000.
The loss of Raqqa would be another major setback for ISIS after the terrorist group was driven from its main Iraqi bastion of Mosul last month.
The terrorists have lost 78% of the territory they held in Iraq and 58% of what they had in Syria, Mr McGurk added.
"Today in Raqqa, Isis is fighting for every last block... and fighting for their own survival," he said, using another acronym for ISIS. "They most likely will die in Raqqa."
More than 300,000 people have lost their lives in six years of conflict in Syria, which began with protests against President Bashar al-Assad before escalating into a full-scale civil war. Eleven million people have been displaced by the fighting.