Fourteen patients and medical staff are among at least 27 people killed in overnight air strikes on a hospital supported by Doctors Without Borders in Syria, the aid group said.
Six hospital staff and three children were among the dead after Quds hospital and nearby areas in the rebel part of the contested city of Aleppo were attacked.
The air strikes came as the UN envoy for Syria Staffan de Mistura appealed to the US and Russia to help revive the peace talks and a ceasefire, which he said "hangs by a thread".
Chief Syrian opposition negotiator Mohammed Alloush blamed the government of President Bashar Assad for the deadly attacks.
He said the latest violence by government forces shows "the environment is not conducive to any political action".
The Syrian Civil Defence, also known as the White Helmets, said the hospital and adjacent buildings were struck in four consecutive air strikes.
It said there were still victims buried under the rubble and that the rescue work continued.
A video posted online by the White Helmets showed a number of lifeless bodies, including those of children, being pulled out from a building and loaded into ambulances amid screaming and wailing.
It also showed distraught rescue workers trying to keep onlookers away from the scene, apparently fearing more air strikes.
Doctors Without Borders, also known as MSF, tweeted: "Destroyed MSF-supported hospital in Aleppo was well known locally and hit by direct air strike on Wednesday."
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 Mr 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 said.
"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 White Helmets, leaving five of its team dead in rural Aleppo.
Nearly 200 people have died since April 19, including at least 44 in an air strike on a market place in a rebel-held area in northern Idlib province, as well as dozens of civilians in government-held areas from rebel shelling.