United States-backed forces said on Tuesday that they had captured Raqa's state hospital, one of the last redoubts of the militant Islamic State group fighters holding out in their onetime Syrian bastion.

“The national hospital was liberated and... 22 foreign mercenaries were killed,” the Syrian Democratic Forces (SDF) said.

“Clashes continue with great intensity near the municipal stadium,” the other main position still in IS hands, it said.

The advance came a day after the SDF took an infamous roundabout used by the militants for public beheadings and crucifixions in Raqa.

The SDF, backed by a US-led coalition, are on the verge of seizing Raqa after having taken around 90 per cent of the group's former de facto Syrian capital.

As of late Monday, only around 300 IS fighters were believed to remain in the devastated city.