US-backed forces struggled to gain ground against the Islamic State group in Syria's Raqa today, facing a wave of car bombs and mortars unleashed by the jihadists, a monitor said.
The Syrian Democratic Forces penetrated Raqa's Old City earlier this week after US-led coalition air strikes pierced two holes in its ramparts.
But the SDF and a unit of allied Arab fighters called the Syrian Elite Forces have made little progress inside the historical quarter, the Syrian Observatory for Human Rights monitoring group said.
"IS is using car bombs, mortar fire, and snipers to wage a counter-offensive inside the Old City," Observatory head Rami Abdel Rahman said.
The Observatory estimates that up to 30 per cent of Raqa has been captured by the US-backed forces since they entered the city on June 6.
Mohammad Khaled Shaker, a spokesman for the Syrian Elite Forces, told AFP there had been no notable advance today.
"There are some clashes, but we have not reached the city centre yet," he told AFP.
IS overran Raqa in early 2014 and it has since become infamous as the de facto Syrian capital of the group's so- called "caliphate".
An estimated 2,500 jihadists are left defending Raqa, according to the US-led coalition.
The global alliance is providing the SDF with air cover, weapons, equipment, and special operations forces on the ground as advisers.
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