Rebels have reportedly shelled a mosque in a government-held neighbourhood of Aleppo, killing at least 15 people as they left Friday prayers.
The attack came as government air strikes hit rebel-held parts of the city, killing at least six people, even after the Syrian army unilaterally declared a brief truce in other parts of the country.
The violence in Aleppo has killed more than 200 civilians over the past week and is likely to continue unchecked, as the government's cease-fire does not include Aleppo.
At least 15 people were killed and 30 injured when rockets struck Malla Khan mosque in the government-held Bab al-Faraj district shortly after Friday prayers, Syrian state TV reported.
"We want the army to finish them," an Aleppo resident told state TV outside a hospital where some of the dead and wounded were taken.
The attack followed an early-morning lull in government air strikes on rebel-held parts of Aleppo, following days of deadly violence that killed scores of people.
Yet by late morning air raids resumed on the city, according to activists from the Britain-based Syrian Observatory for Human Rights and the Local Co-ordination Committees (LCC).
The resumption of air raids prompted religious leaders to suspend Friday prayers in rebel-held areas.
The Religious Council of Aleppo, a body that runs religious affairs in opposition-held parts of the province, said in a statement online that it was calling on mosque leaders to call off Friday prayers for the first time.
"The heart of the believers is aching ... but preserving lives is an important religious duty," it said.
The Observatory reported that six people were killed in the strikes on opposition areas, while the LCC said at least 10 people died and dozens were wounded.
The LCC and the Observatory also reported shelling near a clinic in the rebel-held neighbourhood of Marjeh. There was no word on casualties.
The humanitarian situation has deteriorated further in opposition-held neighbourhoods, which are almost encircled.
The sole road leading in and out of rebel-held areas from north-west Aleppo is regularly targeted by sniper fire and shelling.
The carnage in Aleppo - a city contested since the summer of 2012, when opposition fighters took over several districts - was particularly bad on Wednesday and Thursday, when air strikes and artillery killed more than 60 people, including dozens at a hospital.
Also on Friday, the Syrian army declared a temporary truce for the capital Damascus, its suburbs and the coastal province of Latakia - but not Aleppo.
It was not immediately clear what impact this unilateral declaration would have.
The opposition seemed unlikely to abide by the truce after dozens were killed in the latest air strikes.
The cease-fire is due to come into effect at 1am on Saturday, according to an announcement read on Syrian state TV.
The military statement said it will last 24 hours in Damascus and its suburbs and three days in Latakia.
Yet in Aleppo, which has once more become the civil war's main battleground, aid agencies warned of a potential humanitarian disaster following the collapse of a two-month US and Russia-brokered cease-fire and the stalling of peace talks in Switzerland.
In Geneva, the UN High Commissioner for Human Rights said the latest reports of civilian deaths in Syria revealed a "monstrous disregard for civilian lives by all parties to the conflict".
In a statement released on Friday, Zeid Ra'ad Al Hussein urged all sides to step back from a return to an all-out war.
The UN official said "the violence is soaring back to the levels we saw prior to the cessation of hostilities" in late February.
He added that targeting medical facilities and markets could "amount to war crimes".
According to the Observatory, air strikes and shelling in Aleppo killed 202 civilians in the past week - 123 in rebel-held parts of the city and 71 and government-held areas.
It said the dead included 31 children on both sides.
Doctors Without Borders later said the death toll from the bombing of the hospital and nearby buildings in Aleppo has risen to 50, including six medical staff and patients.
The organisation, also known by its French acronym MSF, said the bombing of Quds hospital late on Wednesday destroyed one of the last remaining places in Aleppo in which "you could still find humanity".
MSF warned that the 250,000 residents in the rebel-held parts of Aleppo are in danger of being completely cut off and left without medical care.
Air strikes on the hospital drew international condemnation.
Muskilda Zancada, head of MSF's Syria mission, said "the sky is falling in Aleppo".