At least 20 people, including two doctors and three children, were killed when a series of air strikes hit a hospital and nearby buildings in the rebel-held part of Syria's contested city of Aleppo overnight.
The attack came as the UN envoy for Syria appealed to the US and Russia to help revive the Syrian peace talks and a ceasefire he said "hangs by a thread".
The chief Syrian opposition negotiator, Mohammed Alloush, blamed the government of President Bashar Assad for the deadly air strikes on Aleppo. He told the Associated Press that the latest violence by government forces shows "the environment is not conducive to any political action".
The strikes hit shortly before midnight on Wednesday, according to opposition activists and rescue workers. They struck a well-known field hospital in the rebel-held district of al-Sukkari in Aleppo. The dead included one of the few paediatricians remaining in the city's opposition-held areas and a dentist, activists said.
The head of the Britain-based Syrian Observatory for Human Rights said at least 20 people were killed, including three children, and that the hospital was completely destroyed.
The Syrian Civil Defence, a volunteer first-responders agency, whose members went to the scene of the attack, said the al-Quds hospital and adjacent buildings were struck in four consecutive air strikes.
The agency, also known as the White Helmets, gave a slightly higher toll, saying 22 were killed. It said there were still victims buried under the rubble and that rescue work was continuing.
Among those killed were three of the hospital's medical staff, they said.
Mr Alloush, who was one of the leading negotiators of the opposition in the Geneva talks, described the air strikes as one of the latest "war crimes" of Assad's government.
"Whoever carries out these massacres needs a war tribunal and a court of justice to be tried for his crimes. He does not need a negotiating table," he told the AP in a telephone interview. "Now, the environment is not conducive for any political action."
The February 27 ceasefire has been fraying in the past weeks as casualty figures from violence mount, particularly in Aleppo and across northern Syria. Air strikes earlier this week also targeted a training centre for the Syrian Civil Defence, killing five of its team in rural Aleppo.
Since April 19, nearly 200 people have died, including at least 44 in an air strike on a market place in rebel-held area in northern Idlib province, as well as dozens of civilians in government-held areas from rebel shelling.
The UN envoy for Syria, Staffan de Mistura, briefed the UN Security Council via video-conference about the largely stalled indirect talks between the Western- and Saudi-backed opposition and envoys from Assad's government, which has the backing of Moscow.
He said that, after 60 days, the cessation of hostilities agreed to by both sides "hangs by a thread".
"I really fear that the erosion of the cessation is unravelling the fragile consensus around a political solution, carefully built over the last year," he said in his council briefing obtained by the Associated Press.
"Now I see parties reverting to the language of a military solution or military option. We must ensure that they do not see that as a solution or an option."
The talks foundered last week after the main opposition group, called the High Negotiating Committee, suspended its formal participation in the indirect talks with Assad's envoys to protest alleged government ceasefire violations, a drop in humanitarian aid deliveries and no progress in winning the release of detainees in Syria.