BEIRUT (Reuters) - U.S.-backed forces advanced inside the Islamic State-held city of Manbij in northern Syria on Friday, seizing a wheat silo complex on its outskirts and a roundabout 2 km (1 mile) south of its centre, a monitoring group said.

The Syria Democratic Forces, comprised of Kurdish and Arab fighters and backed by the air power of a U.S.-led coalition to fight IS, were able to overlook more than half the city from the silos, the Syrian Observatory for Human Rights said.

The advance took place more than three weeks after the SDF and the coalition launched an offensive to drive the jihadist militants from territory it controls along the Turkish-Syrian border. They had rapidly advanced to just outside the city and cut IS supply lines leading there.

The U.S. coalition has said it expects a tough fight from IS militants inside the city. The SDF had refrained from entering out of concern for civilians, many of whom have fled in recent weeks.