The UN envoy to Syria has urged the US and Russia to urgently intervene “at the highest level” to salvage talks.
Speaking after briefing the UN Security Council on the faltering peace process, Staffan de Mistura said that a truce agreed in February was “barely alive”.
Violence in Syria has intensified in recent days, despite the ceasefire.
At least 20 civilians were reportedly killed on Wednesday in government strikes on a hospital and nearby residential building in eastern Aleppo.
Civil defence volunteers, known as the White Helmets, told French news agency AFP that the dead included children and the only paediatrician left in rebel-held areas of the city.
Calling on the US and Russia to cooperate, Mr de Mistura said that the legacies of both President Barack Obama and President Vladimir Putin were linked to the success of the peace process in Syria.
Mr de Mistura said that the fragile “cessation of hostilities” established in February had been “saved from total collapse” but “could collapse any time”.
He said that over the past 48 hours an average of one Syrian had been killed every 25 minutes and one wounded every 13 minutes.
In order for the peace talks to succeed, Mr de Mistura said, hostilities would need to be again reduced to the levels seen immediately following the February truce agreement.
His press briefing followed the third session of talks this year between the major actors in the conflict.
The main opposition delegation, known as the High Negotiations Committee (HNC), last week suspended its role in the talks to protest against alleged government ceasefire violations and a drop in humanitarian aid to besieged areas.
Asked whether the role of President Bashar al-Assad in a transition government was discussed in the most recent round of talks, Mr de Mistura said the parties “didn’t get into names of people, who is doing what, but about how to change the current governance”.
There will be one or two more rounds of talks before July, Mr de Mistura said. He said that the latest round had been “overshadowed by a substantial and worrisome deterioration of hostilities.
“We cannot ignore that and we have not ignored it,” he said.
He added: “There are still major differences on the major issues, but there is movement on certain areas where there was not before.”
Mr de Mistura also said that equal rights and equal representation in major institutions for women was essential to the transition to a new Syria.
The Syrian Observatory for Human Rights monitoring group confirmed on Wednesday that the airstrikes on a hospital and nearby buildings in Aleppo were carried out by “regime airplanes”.
Volunteer rescuers told AFP that a dentist and five members of a family, including two children, were among those killed by the strikes in the al-Sukkari neighbourhood.
Separately, 11 people were killed in western Aleppo earlier on Wednesday, the Syrian Observatory said.
More than 270,000 people have been killed since Syria’s bitter civil war conflict erupted in 2011 and millions have been forced to flee.