BEIRUT (REUTERS) - Islamic State militants have withdrawn from areas north of the Syrian city of Homs and have retreated east after coming under attack from rival Islamist fighters, a monitoring group said on Sunday.

Islamic State (ISIS), an Al-Qaeda offshoot, has advanced often unhindered across Syria and neighbouring Iraq in recent months, so any sign that its opponents are successfully fighting back is significant.

Fighters from the group withdrew from a headquarters north of Homs on the orders of their leader Abu Bakr al-Baghdadi, the Syrian Observatory for Human Rights said, citing sources in areas north of Homs.

ISIS gave up the territory to Nusra Front, Al-Qaeda's official wing in Syria, according to the Observatory, which tracks violence in Syria through sources on the ground.

As well as Nusra Front, Western-backed rebels have also been fighting ISIS in Syria but have regularly been defeated by the group, which in June declared an "Islamic caliphate" in the territory it controls.

ISIS has also clashed with forces loyal to Syrian President Bashar al-Assad in strategic areas. In late July, the Syrian army recaptured a gas field east of Homs that ISIS militants had seized earlier that month in a raid killing some 270 soldiers, guards and staff, the Observatory said.

In north-east Syria, ISIS is battling to wrest control from the Syrian army of a military airbase east of the city of Raqqa. The airbase is the government's last foothold in the area.

On Sunday, army jets bombed areas around the airport, the Observatory said.

ISIS has made rapid gains in Syria, mostly by seizing territory from rival rebel groups using weaponry brought in from Iraq where it has also managed to take large areas from government forces.

Activists have accused the Syrian army of avoiding confrontations with ISIS because it has weakened rival rebel groups also battling Mr Assad.