Syrian opposition activists and militant websites say a push by an alliance of rebel groups and al-Qaida-linked militants to open a new front with government forces in Aleppo has failed.

The activists and the websites reported on Monday that the rebels and militants retreated from positions they seized near a cement factory a day earlier on the southwestern outskirts of the city.

The attempt was spearheaded by Syria's rebranded al-Qaida branch, which now goes under the name of Fath al-Sham. It was formerly known as the Nusra Front.

Fierce fighting began on Sunday and lasted for hours around key army positions at the cement factory. Aleppo has become the focal point of the war.

The Syrian Observatory for Human Rights said at least 35 rebels were killed in the fighting.