"Our forces today seized full control of the Old City in Raqa after clashes with Daesh," Syrian Democratic Forces spokesman Talal Sello said, using the Arabic acronym for ISIS.
"We are on the edges of ISIS' security quarter in the city centre, where most of its main bases are."
Most of ISIS' fighters and up to 25,000 civilians are expected to still be holed up in the city centre.
The SDF's Arab and Kurdish fighters first broke into Raqa in early June and penetrated its Old City a month later.
They were able to enter it after US-led coalition air strikes opened up two gaps in the Rafiqah wall, a 2,500-metre (2,735-yard) barrier surrounding the Old City.
The SDF has captured more than 60 per cent of Raqa city and most of the surrounding northern province.
"Control over the Old City -- which has historical importance -- is a moral victory against ISIS, which is collapsing in Raqa. Its defeat there is inevitable," he added.
The Syrian Observatory for Human Rights, a Britain-based monitoring group, said the SDF was still locked in clashes with ISIS in a small part of the Old City.
ISIS overran Raqa in 2014, turning it into the de facto capital of its self-declared "caliphate".
The city was the scene of some of the group's worst atrocities, including public beheadings.
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