The US-backed Syrian Democratic Forces (SDF) deployed around a thousand additional fighters to the front lines with Islamic State in Raqqa on Sunday, the SDF media office said.
The fighters were sent to the east and west of the city, Islamic State's de facto capital in Syria. The SDF launched its US-backed assault to capture Raqqa last month.
They had "completed their training in SDF camps in cooperation with the coalition forces, with the aim of supporting the campaign and achieving new progress", the SDF media office said in a statement, citing SDF commanders.
The Syrian Observatory for Human Rights, a UK-based organization that reports on the war, said the SDF had made advances on Sunday in the south of the city, and had also retaken ground lost in an Islamic State counterattack on Friday.
"The clashes are extremely violent," Observatory Director Rami Abdulrahman said.