DAMASCUS, Syria (AP) — Air strikes and shelling pounded Aleppo for the third straight day Sunday, killing two young siblings and at least 24 others in Syria's largest city and former commercial capital.

The northern city has been bitterly contested between insurgents and government forces since 2012. Opposition groups control the eastern part of the city but have come under intense strain as the government has choked off all routes to the area except a narrow and perilous passage to the northwest.

At least 10 people were killed by rebel shelling on government-held areas in the city, according to activists and Syria's state news agency, SANA. Rockets struck schools and residential areas, SANA reported. The Britain-based Syrian Observatory for Human Rights said two young siblings were among the dead.

Air strikes on the opposition side of the city killed 16, including a mother and her daughter, the Observatory said. The activist-run Local Coordination Committees network blamed the strikes on the government.

The Aleppo Conquest rebel coalition on Saturday threatened to dissolve a nearly two-month-old cease-fire with the government if pro-government forces continued to strike civilians in opposition areas.

The U.S. and Russia-brokered cease-fire is still technically in place, but may have completely unraveled on the ground — with violence returning to most of the contested areas of the country. The U.N.'s Special Envoy to Syria last week called on the two superpowers to salvage the truce before it totally collapses.

The al-Qaida branch in Syria, the Nusra Front, and its more powerful rival, the Islamic State group, are not included in the cease-fire. The Nusra Front is deeply rooted in the areas in northern Syria controlled by opposition forces, complicating the oversight of the truce.

U.N.-mediated talks in Geneva have also been bogged down by the violence, with the Saudi-backed opposition delegation suspending its formal participation last week. The government delegation is nonetheless set to meet with U.N. envoy Staffan De Mistura Monday.

Opposition groups have said reports of a new government offensive on the opposition-held side of Aleppo would wreck the peace talks.

Issa reported from Beirut.