Syria’s partial ceasefire is unravelling as fierce fighting between government forces and opposition fighters, including members of the al Qaida affiliated Nusra Front, erupted outside the country’s second largest city of Aleppo.

At least 25 pro-government and 16 opposition fighters died in clashes south of Aleppo, where the Nusra Front and rebel militias captured a hill overlooking a major highway.

The Syrian Observatory for Human Rights said fighting continued throughout the day close to the village of Tel al-Ais, which overlooks the main road connecting Aleppo with the capital, Damascus.

The co-ordinated rebel and Nusra Front offensive follows weeks of air raids on opposition-held areas despite a “cessation of hostilities” that came into effect late in February.

The truce agreement, the first of its kind in Syria’s five-year war, excludes the Nusra Front and the Islamic State group.

But the Nusra Front is embedded with other groups throughout the country and the government has taken advantage of this to strike and besiege opposition-held areas across Syria.

Bombs fell near a school and a hospital in the eastern suburbs of Damascus on Thursday, killing 33 civilians.

Opposition officials, accusing the government, said the “massacre” threatened to derail the peace talks scheduled to resume in Geneva in two weeks.

The US and Russia, who engineered the ceasefire agreement, had hoped a halt in fighting would cause opposition factions to distance themselves from extremist groups such as the Nusra Front.

Instead, factions seem to have united in their opposition to the government.