Fourteen patients and medical staff are among those killed in overnight air strikes on a hospital supported by Doctors Without Borders in Syria, the aid group said.
Opposition activists and rescue volunteers said the death toll from the strikes that hit the Quds hospital and nearby areas in the rebel part of the contested city of Aleppo rose to at least 27 on Thursday.
The Syrian Civil Defence, a volunteer first-responders agency, said six hospital staff were among the dead, including one of the few paediatricians left in opposition-held areas of Aleppo.
The group put the total death toll at 30 while the UK-based Syrian Observatory for Human Rights said at least 27 were killed.
At least 20 people have been killed in a series of air strikes in northern Syria which hit a hospital and nearby buildings in the city of Aleppo, a Syrian monitoring group and rescue workers said.
The victims included three of the hospital's medical staff.
The head of the Britain-based Syrian Observatory for Human Rights said the air strikes took place shortly before midnight on Wednesday and that at least one child was among the 20 killed.
The Syrian Civil Defence, volunteer first-responders in rebel-held areas, gave a higher toll, saying the strikes on the Quds hospital killed 22.
The dead included one of the few remaining paediatricians in opposition-held areas of the contested city.
The Syrian Civil Defence said there were four consecutive air strikes against the hospital and adjacent buildings.
Earlier, the UN envoy for Syria appealed to the US and Russia to intervene to help revive peace talks.
Staffan de Mistura said a recent spike in fighting had overshadowed the talks and put an increasingly fragile ceasefire in "great danger".
Mr de Mistura was speaking after briefing the UN Security Council about the stalled indirect talks between the Western and Saudi-backed opposition and envoys from President Bashar Assad's government, which has the backing of Moscow.
Mr de Mistura said he hoped the talks would resume in May and predicted the process would continue until July.
Criticising the violence, he said: "In the last 48 hours, we have had an average of one Syrian killed every 25 minutes, one Syrian wounded every 13 minutes ... How can you have substantial talks when you have only news about bombing and shelling?"
Speaking at the end of a third session of Syria peace talks this year, Mr de Mistura said a truce brokered by the US and Russia had pulled off a "miracle" by sharply reducing violence in March, but acknowledged that the renewed fighting has put the ceasefire "in great danger".
He called for a "US-Russian initiative at the highest level" to help reinforce it.
"There is no reason that both of them - who have been putting so much political capital in that success story and have a common interest in not seeing Syria ending up in another cycle of war - should not be able to revitalise what they created, and which is still alive, but barely," he said.
The talks stalled last week after the Western and Saudi-backed opposition, the High Negotiations Committee, suspended formal participation in the talks with Assad's envoys in protest at alleged government ceasefire violations, a drop in humanitarian aid deliveries and no progress in winning the release of detainees in Syria.
Mr de Mistura suggested work between Moscow and Washington is already under way to reinforce the truce and a revival of it would help bring the two sides back to the table.
"I know that both the Russian Federation and the US are talking among themselves on how to salvage on what has been actually a remarkable success - but needs to be sustained," he said.
If it is revived, he predicted: "It will not be difficult for everyone to come back around the table."
Mr de Mistura said that, after 60 days, the ceasefire "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. "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 current effort to end the five-year Syria conflict was largely spearheaded by US Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov, backed by major global and regional powers who formed the International Syria Support Group. It includes 17 countries as well as the United Nations, Arab League, European Union and Organisation of Islamic Co-operation.