At least 30 civilians were killed Saturday in regime and rebel bombardment of areas across Syria, threatening an eight-week-old truce at a time when peace talks are stalled in Geneva.
The head of a Britain-based monitoring group said the escalating violence meant a ceasefire between the regime and non-jihadist rebels, in place since late February, had effectively collapsed.
The truce brokered by Russia and the United States had raised hopes that UN-backed talks in Geneva this month would lead to a solution to the five-year conflict.
But the negotiations due to continue until Wednesday have faltered after Syria's main opposition group this week suspended its official participation in the talks.
Twelve civilians were killed in air strike on the northern metropolis of Aleppo on Saturday, a local civil defense official said.
State news agency SANA, meanwhile, said three civilians were killed and 17 wounded in rebel shelling of government-held areas of Aleppo.
The Syrian Human Rights Observatory said 13 others died in shelling of the rebel-held town of Douma, east of Damascus, while two men were killed in regime air strikes on Talbisseh in central Homs province.
The barrage of air strikes on Aleppo targeted several neighborhoods, including the heavily populated Bustan al-Qasr district, an AFP correspondent in the city said.
The deadliest raid was on the Tariq al-Bab neighborhood on the eastern edges of the city.
A civil defense volunteer was seen carrying a screaming woman down a ladder from a damaged building in the neighborhood, as a pick-up truck removed the remains of a victim's body.
Another volunteer operating a crane brought down a young man cradling a baby from an upper storey.
It was the second day of deadly strikes on Aleppo, after 25 civilians were killed and another 40 wounded in air strikes on Friday.
Regime "intensifying air strikes"
Once Syria's commercial hub, the northern metropolis has been divided by government control in the west and opposition groups in the east.
"The ceasefire ended when the first bomb hit the city," Mohammed Mashhad, a civil defense volunteer, said.
"The regime is intensifying its air strikes, which have reached around 20 a day," the 42-year-old said.
"This regime is criminal and doesn't understand the language of political negotiations. All it gets is bombing, killing and destruction."
In the rebel-held town of Douma, 13 people - including three women and two children - were killed in government shelling on the city. The Observatory said all the dead were civilians.
Douma lies in the Eastern Ghouta opposition bastion, where the Jaish al-Islam rebel group - also party to the truce deal - is dominant.
The ceasefire deal saw Syria's government and non-jihadist opposition agree to halt attacks while pursuing peace talks.
Violence dropped across the country, including in Aleppo city, where residents cautiously began shopping in open-air markets and taking their children to parks.
But Observatory director Rami Abdel Rahman said on Saturday that the truce had effectively collapsed.
"Most of the areas that were under the ceasefire are now seeing fighting again," he said.
US President Barack Obama and the UN's special envoy to Syria Staffan de Mistura on Friday said the ceasefire was in grave peril.
Syria's main opposition High Negotiations Committee halted its formal participation this week in the Geneva talks, which started on April 13.
But De Mistura said members of his team had continued to meet remaining HNC members at their Geneva hotel.
In Syria itself, regime officials and Kurdish representatives were to meet Saturday for a second day of talks aimed at ending deadly clashes in the northeastern city of Qamishli, a senior security source said.
More than 270,000 people have been killed since Syria's conflict broke out in 2011.
AFP contributed to this report.