The US-backed Syrian Democratic Forces militia (SDF) began a campaign to capture Raqqa from IS last year, slowly encircling the city before breaking into it for the first time in June.
"As of today, the SDF has seized about 45 percent," of Raqqa, said Brett McGurk, the senior US envoy to the international coalition fighting IS in Iraq and Syria.
Major milestone
The recapture of Raqqa would mark a major milestone in the three-year effort to defeat IS.
The jihadists were ousted from their main Iraqi bastion of Mosul last month.
Still, the Raqqa battle is far from over, with thousands of IS fighters remaining.
"I'm always hesitant to give numbers like that because this is an inexact science, but we think there's about 2,000 fighters left in Raqqa -- and they most likely will die in Raqqa," McGurk said.
The United Nations estimates between 20,000 to 50,000 civilians may still be in the city, though other estimates are lower.
