UN Special Envoy says one Syrian civilian has been killed every 25 minutes over the last 48 hours
Syria's fragile ceasefire is hanging by a thread after airstrikes on an Aleppo hospital killed at least 27 people, including a doctor believed to be the city's last paediatrician.
It comes as the UN called on the US and Russia to salvage the "barely alive" two-month truce and make urgent efforts to revitalise the peace process.
The four consecutive strikes hit Al Quds hospital and nearby homes in the rebel-held district of Sukkari just before midnight on Wednesday.
Three children are believed to be among the dead, with Doctors Without Borders - which helps run the hospital - saying 14 patients and medical staff had died.
The UK-based Syrian Observatory for Human Rights put the total number of dead at at least 27.
Volunteer rescue team The White Helmets are continuing to dig through the debris in an attempt to locate and recover survivors.
A tweet from the Syrian Coalition said the number of dead could rise significantly.
Mohammed Alloush, the political leader of the country's Saudi-backed rebel group Jaysh al-Islam (Army of Islam), and one of the leading opposition negotiators at February's peace talks in Geneva, blamed the attack on President Bashar al Assad.
"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 ceasefire brokered by the US and Russia in February has been crumbling, as casualty figures from government and opposition violence escalate in Aleppo and across northern Syria.
Since mid-April some 200 people are reported to have been killed, including at least 44 in an airstrike on a market in a rebel-held area of Idlib and dozens of civilians in government-held areas during rebel shelling.
UN Special Envoy Staffan de Mistura expressed concern the truce was beginning to unravel in rebel-held areas and urged a new US and Russian initiative to save the peace process.
Speaking after a briefing to the UN Security Council, he said one Syrian civilian has been killed every 25 minutes over the last two days, with one wounded every 13 minutes.
Mr de Mistura called on the US and Russian leaders to help salvage the 'barely alive' ceasefire in Syria, Reuters reported.
"The legacy of both President Obama and President Putin is linked to the success of what has been a unique initiative which started very well. It needs to end very well," he said at a UN news conference in Geneva.
The two leaders are being urged to convene a meeting of the International Syria Support Group (ISSG) - a group consisting of both international and regional leaders.
More than 270,000 people have been killed since the outbreak of the civil war in Syria in 2011.
The fragile ceasefire, which has been in place since February, has marked the first major cessation of hostilities in the conflict.
A third round of Syrian peace talks have taken place, with Mr de Mistura suggesting the next round can only be 'meaningful' if the level of violence decreases again.
"How can you have substantial talks when you have only news about bombing and shelling? It's something that even I find it difficult - can you imagine the Syrians?" Mr de Mistura added.