More than 20 people were killed in the Syrian city of Aleppo on Friday, bringing the death toll to at least 200 civilians in the past week.
Rebels shelled a mosque, killing at least 15 people and injuring 30 as they left Friday prayers, Syrian state TV reported.
"The heart of the believers is aching... but preserving lives is an important religious duty," it said.
The Britain-based Syrian Observatory for Human Rights reported that six people were killed in the strikes on opposition areas, while the Local Co-ordination Committees (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.
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.
The dead included 31 children on both sides.