Road under sniper fire in the souk of Aleppo in early March. This area is now contested between the government forces and the opposition. (Lorenzo Tugnoli for the Washington Post)

Syria’s partial cease-fire is unraveling, as fierce clashes between government forces and opposition fighters, including members of al-Qaeda-affiliated al-Nusra Front, erupted Saturday outside the 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 Britain-based Syrian Observatory for Human Rights said. The fighting continued throughout the day Saturday close to the village of Tel al-Ais, which overlooks the main road connecting Aleppo with the capital, Damascus.

The coordinated offensive by rebels and the Nusra Front follows weeks of air raids on opposition-held areas despite a “cessation of hostilities” that came into effect in late 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. The government has taken advantage of this ambiguity 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 a reported 33 civilians. Opposition officials, accusing the government, said the “massacre” threatened to derail the peace talks that are scheduled to resume in Geneva in two weeks.

Government airstrikes also targeted the public square in the opposition-held city of Maarat Nouman in northern Idlib province Friday, where residents had protested against the Nusra Front’s presence in the town.

These latest strikes appear to have caused some rebel factions to reassess their positions.

The Islam Army, whose political coordinator heads the opposition delegation during fitful peace talks in Geneva, announced that it had killed 20 government soldiers in fighting outside Damascus on Friday.

A spokesman for a U.S.-backed division of the Free Syrian Army accused the government of scrapping the cease-fire and undermining the talks in Geneva. The group said one of its fighters was killed in the offensive against government forces in the south Aleppo countryside.