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.

(Reporting by John Davison, editing by Larry King)