Air strikes overnight on the Medecins Sans Frontieres (MSF)-backed al Quds hospital and a nearby residential building in the opposition-held Syrian city of Aleppo killed at least 47, including 27 patients, doctors and staff, the Syrian Observatory for Human Rights (SOHR) said on Thursday.
The death toll is expected to rise due to the number of serious injuries.
Three doctors, one of whom was the last paediatrician in the city, were among those killed, the SOHR reported.
One source working for the forces of the Syrian regime said that regime warplanes had not been used in the areas where air strikes were reported.
— MSF International (@MSF) April 28, 2016
The United Nations said on Thursday that the situation in Aleppo is "catastrophic" after deadly overnight air strikes on the hospital, and that aid deliveries to millions of Syrians are in jeopardy.
An aid convoy was hit by a mortar round near Homs this week while another was forced to stop due to air raids, said Jan Egeland, chairman of the UN humanitarian task force for Syria.
"The stakes are so incredibly high because so many civilian lives are at stake, so many humanitarian health workers and relief workers are being bombed, killed, maimed at the moment that the whole lifeline to millions of people is now also at stake," Egeland told reporters in Geneva.
"Doctors have been killed, health workers have been killed and medical workers have been blocked from coming to their patients," he said, speaking after a weekly meeting of major and regional powers in the International Syria Support Group (ISSG).
Egeland cited a "catastrophic deterioration in Aleppo over the last 24-48 hours" as well as in parts of the Homs area.
Aleppo has been the epicentre of a military escalation that has helped to undermine UN-led peace talks in recent weeks. A cessation of hostilities agreement has unravelled and fighting has resumed on numerous fronts in western Syria.
The city is divided into areas held by the government and opposition forces.
In the past six days, 91 civilians have been killed in regime air strikes in Aleppo, according to the SOHR.
The UN said on Thursday that the February 27 cessation of hostilities deal is "barely alive."
Around 400,000 people have been killed in Syria's five-year war, according to UN Special Envoy to Syria Staffan de Mistura.