At least 60,000 people have fled towns in the Deir Ezzor province in eastern Syria which has been the scene of fierce clashes between rival rebel groups, opposition activists say.

The al-Qaeda affiliated al-Nusra Front have been battling the Islamic State of Iraq and the Levant (ISIL) for four days despite an order from al-Qaeda chief Ayman al-Zawahiri to stop fighting, the UK-based Syrian Observatory for Human Rights said on Sunday.

"Residents of the towns of Busayra, home to 35,000 people, Abriha, home to 12,000 people, and al-Zir, home to 15,000 people, have nearly all been displaced by the fighting in the area," the Observatory said.

The group, which relies on a vast network of contacts on the ground for its data, said al-Nusra Front's fighters burnt down several houses in Busayra, as did ISIL in Abriha.

At least 62 fighters have been killed in this week's clashes, said the Observatory. "There are battles now in an area around 10km from Busayra that has an oil rig and a gas plant," Observatory director Rami Abdel Rahman said.

The latest showdown between al-Nusra and ISIL erupted on Wednesday in energy-rich Deir Ezzor, bordering Iraq.

Homs evacuation delayed

Meanwhile, the planned evacuation of fighters from rebel-held parts of the Syrian city of Homs has been delayed by a day, activists say, though a ceasefire is still holding in the country's third-largest city.

A rebel pullout from Homs, which is known as the 'capital of the revolution', would hand President Bashar al-Assad complete control of the city and deal a major symbolic blow to the uprising-turned civil war.

But local activists said it wasn't clear why Syrian forces weren't allowing the first phase of several hundred rebel fighters to leave the Old City area on Saturday.

Rebels in the city agreed on Friday to surrender territory in exchange for safe passage to other opposition-held areas. Homs is strategically important as it connects government strongholds along the western coast with the capital, Damascus.

The conflict in Syria since March 2011 has killed more than 150,000 people and forced nearly half the country's population to flee their homes.