A member of the Syrian Democratic Forces checks a building near the central hospital of Raqa on October 1, 2017

US-backed fighters on Monday combed through a central district of Syria's Raqa that once housed key Islamic State group command centres, recovering communication equipment and weapons used by the jihadists.

The Syrian Democratic Forces have captured around 90 percent of IS's one-time bastion Raqa, including about half of the Al-Nahda neighbourhood on the western edge of the city centre.

After hard-fought battles against IS this week, SDF units on Monday were searching house-by-house for weapons or documents left behind by IS.

"This neighbourhood was a strategic place for Daesh. Most of their bases were here, including command centres and ammunition depots," said local SDF commander Gabar Derek, using the Arabic acronym for IS.

Many of the buildings in the area are large stone complexes formerly used by Syria's government, then taken up as bases by IS before the SDF moved in.

"A few days ago we advanced into a position used by IS and found walkie-talkies. We took out a lot but there was a part we could not remove until today," said Derek, 25.

As he spoke, his fighters unloaded an armoured vehicle full of dozens of walkie-talkies, military fatigues and boots, a gas mask, rocket-propelled grenades, and even a serrated sword retrieved from the former IS command centre.

- All evidence collected -

Arab and Kurdish fighters backed by US special forces in Syria search areas newly taken from the Islamic State group in the central hospital of Raqa on October 2, 2017

"Walkie-talkies are important so we know what channels IS are using to communicate, how they're moving, and what their plans are. We're monitoring them," said Derek, a floral scarf wrapped around his spiky black hair to keep it out of his eyes.

Recent sweeping operations had also uncovered stacks of IS administrative documents, hundreds of mobile phones, a weaponised drone, a tablet computer, and even metal coins branded "the Islamic State".

If the spoils included useful ammunition or military gear, SDF units asked permission to keep them, Derek said.

Fighters on Monday could be seen trying on pairs of beige military boots they had snatched from the IS position.

Everything of potential intelligence value was sent up the chain of command to transfer to the appropriate units.

"Even if we find just a needle, we'll send it to the intelligence," Derek added.

Captured by IS in 2014, Raqa became the de facto Syrian capital of the group's notorious "caliphate" and was believed to serve as a major planning hub, including for attacks abroad.

The SDF -- an alliance of Kurdish and Arab fighters -- are working to tighten the noose on IS jihadists holed up in a part of central Raqa that includes the national hospital and football stadium.

They are believed to be holding civilians in these facilities with them as human shields.