People in Aleppo survey damaged buildings after airstrikes Thursday in the Syrian city. (AP)

The battered Syrian city of Aleppo faced another wave of airstrikes and shelling Friday, activists said, killing worshipers at a mosque and damaging a clinic after an earlier air blitz left dozens dead, including patients and staff at a main hospital.

The attacks — apparently carried out by both sides — further eroded efforts to rebuild a cease-fire and halt what a United Nations envoy described as a “monstrous disregard for civilian lives” by all factions in the conflict.

More than 200 people have been killed in the past week in Aleppo by pro-government airstrikes and rebel barrages on regime-held neighborhoods, said the Syrian Observatory for Human Rights, a monitoring group based in Britain.

Doctors Without Borders said the death toll in an airstrike on al-Quds hospital in Aleppo on Wednesday had risen to 50, including six medical staff members and one of the area’s last pediatricians. The hospital was supported by both Doctors Without Borders and the International Committee of the Red Cross, which also condemned the strike.

“The sky is falling in Aleppo,” Muskilda Zancada, head of the Doctors Without Borders mission in Syria, said in a statement Friday. “The city, consistently at the front lines of this brutal war, is now in danger of coming under a full offensive. No corner is being spared.”

On Friday, rebel-fired mortar rounds slammed into a mosque in the government-controlled Bab al-Faraj neighborhood, killing at least 15 people, the Associated Press reported, quoting Syrian state TV. In opposition-held areas of the city, warplanes destroyed a medical clinic. Activists said there were casualties in other strikes across the city.

“The situation is getting worse,” said Ameen al-Halabi, an activist in Aleppo, adding that strikes on rebel-held areas increased Friday. Religious leaders in opposition neighborhoods suspended public Friday prayers at mosques to avoid further casualties, the Religious Council of Aleppo said in a statement.

The U.N. commissioner for ­human rights, Zeid Ra’ad al-Hussein, said Friday that the “violence is soaring back to the levels we saw prior to the cessation of hostilities.” The partial truce began Feb. 27 under an agreement backed by the United States and Russia, which support rival sides in Syria.

“There are deeply disturbing reports of military build-ups indicating preparations for a lethal escalation,” Hussein said in a statement.

Syrian officials have not given details of the apparent offensive on Aleppo, the nation’s largest city and a strategic prize for both the government and rebel forces opposing President Bashar al-Assad.

On Thursday, Russia’s Foreign Ministry denied that the Kremlin had launched any airstrikes in the past few days. Russia on Friday also blamed the al-Qaeda-linked Jabhat al-Nusra for a mortar attack on its abandoned consulate in Aleppo.

In Berlin, meanwhile, German government spokesman Steffen Seibert directly blamed Syrian forces, accusing Assad’s troops Friday of “blatant violation of humanitarian law” for the hospital strike and other attacks.

Rights groups have blamed Assad’s forces for hundreds of attacks on medical facilities and workers over the course of the five-year conflict.

The World Health Organization and UNICEF, the U.N. children’s rights agency, said Friday that such attacks are clear violations of international law.

“We are outraged at the alarming frequency of attacks on health personnel and facilities in Syria,” the WHO and UNICEF said in a joint statement.

“They deserve greater protection,” the statement said of Syria’s medical workers. “Thousands of lives are at stake.”

Brian Murphy in Washington and Zakaria Zakaria in Istanbul contributed to this report.