The assault marked Islamic State's first sustained effort to fight back against the slow advance into Raqqa by the Syrian Democratic Forces (SDF).

A war monitor said Islamic State regained control over most of the industrial district in Syria's Raqqa on Friday, but the alliance fighting it denied that while acknowledging there was a jihadist counter attack in the east of the city.

The assault marked Islamic State's first sustained effort to fight back against the slow advance into Raqqa by the Syrian Democratic Forces (SDF).

The SDF, a U.S.-backed alliance of Kurdish and Arab groups, took the industrial district this month in its biggest gain so far in Islamic State's Syrian capital of Raqqa.

It said on Friday that there had been heavy clashes since late Thursday in east Raqqa, where the industrial district is located, in the areas of al-Rawdha, al-Nahdha and al-Daraiyah.

However, the Syrian Observatory for Human Rights, a Britain-based war monitor, said Islamic State had regained control over most of the industrial area in fierce fighting.

The SDF, on its social media feed, acknowledged there had been intense clashes, but added the whole industrial district was still in its hands and the attack had been thwarted.

On Thursday, the Observatory said the SDF had managed to take the last stretch of the Euphrates' south bank opposite Raqqa, completely encircling Islamic State inside the city.

Since all Raqqa's bridges were already destroyed, and the U.S.-led coalition was striking boats crossing the river, the city had already been effectively isolated since May.

Naser Haj Mansour, a senior SDF official, told Reuters on Thursday he thought it could be “maybe more than a month or a month and a half” before the group took the city. Previous SDF timescales for its war on Islamic State have proven optimistic.

Beyond Raqqa, Islamic State still retains most of the 200km (130 mile) stretch of the Euphrates valley flowing to the border with Iraq. The Syrian army still holds a big enclave in Deir al-Zor, the area's largest city, on which it is slowly advancing from the direction of Palmyra.