Aleppo has been pounded by air strikes and shelling for a third successive day, with two young siblings and at least 24 others killed 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 choked off all routes to the area except a narrow and perilous passage to the north-west.
At least 10 people were reported killed by rebel shelling in government-held areas in the city.
Rockets struck schools and residential areas. 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.
A video posted on social media by the Syrian Civil Defence first responder group, known as the White Helmets and which operates in opposition-held areas, suggests some of the strikes hit a market in the neighbourhood of Sakhour.
Footage showed overturned vegetable carts strewed among the wreckage.
The opposition High Negotiations Committee (HNC), which suspended its formal participation in peace talks with the government in Geneva last week, called the strikes "an attack on the Geneva process that is the only possible pathway to peace."
Salem Meslet, HNC spokesman, called on Russian president Vladimir Putin to hold its Damascus allies to the terms of a US and Russia-brokered cease-fire that parties signed on to nearly two months ago.
"The key to ending these attacks, and to making progress in the talks, lies in Moscow," said Mr Meslet.
The cease-fire is still technically in place, but may have completely unravelled on the ground with violence returning to most of the contested areas of the country.
The UN's special envoy to Syria last week called on the two superpowers to salvage the truce before it totally collapses.
The Aleppo Conquest rebel coalition threatened on Saturday to dissolve the truce if pro-government forces continued to strike civilians in opposition areas.
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.
UN-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 UN envoy Staffan De Mistura on Monday.
Opposition groups said reports of a new government offensive on the opposition-held side of Aleppo would wreck the peace talks.