About 2,000 Daesh fighters are estimated to remain in the Syrian city of Raqqa, fighting for their survival in the face of an offensive by the U.S.-backed Syrian Democratic Forces, a senior U.S. official said on Friday.

Brett McGurk, U.S. special envoy for the coalition against Daesh, said the SDF had cleared about 45 percent of Raqqa since launching an attack in early June to seize Daesh's stronghold in northern Syria.

"Today in Raqqa [Daesh] is fighting for every last block ... and fighting for their own survival" McGurk told reporters.

Some 2,000 Daesh fighters are left in the city and "most likely will die in Raqqa," he said.

The assault on Raqqa coincided with the final stages of a campaign to drive Daesh from the Iraqi city of Mosul, where the terrorists were defeated last month.

McGurk said Daesh has lost 27,000 square miles (70,000 sq 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.

Before every military operation, coalition forces surround the area targeted to make sure Daesh's foreign fighters cannot escape and make their way out of Iraq and Syria, he said.

With the close cooperation of Turkish forces, the entire Syrian-Turkish border was sealed and Daesh can no longer send militants trained in Syria for attacks in Europe and elsewhere, McGurk said.

The coalition has compiled a database of almost 19,000 names of Daesh fighters gathered from cellphones, address books and other documents found on battlefields which it is sharing with the international police agency Interpol, he said.

Daesh is also fighting the forces of Syrian President Bashar al-Assad, who is backed by Russian air power and Iranian-backed militias.

McGurk said "deconfliction" arrangements the U.S. and Russian militaries have made to avoid accidents as they operate separately in Syria were working well despite deteriorating diplomatic relations between the two countries.

President Donald Trump said on Thursday the U.S.-Russian relationship was at "an all-time and very dangerous low," and Russia said new sanctions imposed by Washington meant an end to hopes for better ties with the Trump administration.

"So far we have not seen an effect on our engagement with the Russians when it comes to Syria," McGurk said.