BEIRUT (AP) — U.S.-backed Syrian fighters are clashing with Islamic State militants in the heart of Raqqa, the extremists' self-styled capital, as scores of civilians flee areas controlled by the group.

The Syrian Democratic Forces, aided by the U.S.-led coalition, launched their offensive to capture the city on June 6, and have since taken several areas.

The Britain-based Syrian Observatory for Human Rights says Monday's fighting is concentrated in the southwestern neighborhood of Yarmouk and the Old City.

The SDF says intense fighting is underway in central Raqqa, adding that its fighters have taken positions near a centuries-old mosque.

The Kurdish-run Hawar news agency says some 180 civilians were able to flee areas controlled by IS while the Observatory put the number in the hundreds.