Dozens of Islamic State group fighters have surrendered over the past day in their one-time bastion Raqa, the US-led coalition backing the offensive on the Syrian city said Saturday.

"Within the past 24 hours, approximately 100 ISIS terrorists have surrendered in Raqa, and were removed from the city," the coalition said in an emailed statement to AFP. 

"Foreign fighters are not being allowed to leave Raqa," it said. 

Earlier on Saturday, the Britain-based Syrian Observatory for Human Rights said all Syrian IS jihadists -- numbering around 200 fighters -- had left their former stronghold with their families.

"All Syrian fighters from the Islamic State group left Raqa over the past five days," said Observatory head Rami Abdel Rahman, saying they headed to unknown destinations. 

A Raqa official told AFP that Syrian IS members had surrendered overnight to the US-backed Syrian Democratic Forces (SDF) battling to take full control of the city, without specifying how many.

"They sent a message to the Raqa Civil Council and to the tribal mediators," the official said. 

"Those that surrendered are local, not foreigners -- the foreigners have not handed themselves in yet," he said.

Members of the council -- a provisional administration for the city set up by the SDF -- had been working with tribal leaders throughout the week to try to secure safe passage for civilians. 

Hundreds of civilians have managed to flee the battle-ravaged city, which once served as the de facto Syrian capital of the jihadist group. 

According to Abdel Rahman, up to 150 foreign jihadists remain in the city and negotiations on their fate are still ongoing.

"The foreign fighters are asking to leave in one group towards areas under IS control in Deir Ezzor province," in eastern Syria, he said. 

But a spokesman for the Kurdish People's Protection Units (YPG), which spearhead the SDF, denied on Saturday that any discussions were taking place for the surrender of the city.

"We completely deny any negotiations or deal for the exit of Daesh. Until this very moment, we are fighting Daesh," Nuri Mahmoud told AFP, using the Arabic acronym for IS.

Backed by US-led coalition air strikes, the SDF's Arab and Kurdish fighters have recaptured around 90 percent of Raqa from IS.