A general view shows the south eastern city of Manbij, in Aleppo Governorate, Syria, June 25, 2016. REUTERS/Rodi Said

BEIRUT (Reuters) - U.S.-backed fighters advanced further into the Islamic State-held city of Manbij in Syria on Thursday, as part of a campaign to drive the group from areas near the Turkish border, the Syrian Observatory for Human Rights reported.

The latest advance brought them to within 1 km (less than a mile) from the city center and put them in control of its southwestern neighborhoods, the British-based monitoring group said.

The Syria Democratic Forces, a U.S.-backed alliance that includes Arab and Kurdish fighting groups, has for more than a month pressed the offensive to drive IS from Manbij, backed by U.S.-led air strikes.

Islamic State fighters have put up tough resistance and have staged a number of counter-attacks since the SDF encircled the city and cut off their supply lines.

The offensive would deprive IS of its only major route to the outside world.