US-backed fighters battling to capture Syria's Raqa are being slowed by fierce resistance from the Islamic State group which has heavily mined the city, a militia spokeswoman has told AFP.

The Syrian Democratic Forces, a Kurdish-Arab alliance backed by an international coalition battling IS, entered Raqa city two months ago.

SDF fighters have now captured more than half of the one-time jihadist bastion, Jihan Sheikh Ahmed, spokeswoman for the Wrath of the Euphrates operation to capture the city, told AFP on Tuesday.

But the force's progress has been hindered by IS which is fighting hard to hang on to the de facto Syrian capital of its self-proclaimed "caliphate."

"Our forces are continuing to advance inside Raqa, but slowly," Sheikh Ahmed said.

She said SDF forces had seized nine districts in the city's west and east, while IS was putting up a fierce defence of the seven remaining neighbourhoods. 

"After being besieged from four sides, Daesh has no option but to surrender or die," she said, using an Arabic acronym for IS. 

"That is what has made its reaction fiercer." 

She said the jihadists had laid explosives "in every centimetre" of the city, slowing the SDF's progress.

"Our goal is not simply to liberate territory geographically, but to liberate the civilians in the city," she said.

The fight for Raqa has prompted tens of thousands of civilians to flee, with many wounded by crossfire or IS-planted explosive devices during their escape.

The United Nations has estimated between 20,000 and 50,000 people remain in the city, though others have given lower estimates.

Sheikh Ahmed said SDF forces were also facing IS snipers and weaponised drones, tactics the jihadist group has employed to defend its territories.

She said the jihadists had even staged commando-style attacks within territory recently captured by the SDF.

"Sometimes they infiltrate from behind (the front lines) through tunnels," she said.

But she said the campaign was progressing "with steady steps, no matter how slow".

The SDF is backed by the US-led coalition battling IS in Iraq and Syria, which has supplied weapons, air cover and a limited number of troops.

Sheikh Ahmed praised that support, which she said was growing as the SDF pushed deeper inside Raqa.

The US envoy to the coalition, Brett McGurk, said last week that SDF fighters hold 45 percent of Raqa, and estimated about 2,000 IS jihadists remained in the city.

Source: AFP