Syrian rebels have launched an offensive aimed at breaking a government siege of eastern Aleppo, where the UN estimates some 300,000 people are trapped with dwindling food and medical supplies.

A rebel alliance that includes the Jabhat Fatah al Sham group – which was formerly the al-Qaeda linked Nusra Front – and the Ahrar al Sham group said it had taken army positions in the southwestern government-held parts of the city within the first few hours of launching a battle to break the siege imposed on rebel-held areas.

The Syrian army confirmed the offensive on state media but said its troops had pushed back rebel fighters from an airforce artillery base and denied the rebel alliance had captured the Hikma school.

The Britain-based Syrian Observatory for Human Rights (SOHR), which gathers information from a network of local informants, said rebels and pro-government forces were clashing along several fronts on the outskirts of the divided city.

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