At least 14 patients and three doctors have been killed in an air strike on a hospital in the Syrian city of Aleppo, the charity Medecins sans Frontieres (MSF) says.
Among those killed in the MSF-supported al-Quds hospital was one of the city's last paediatricians,
MSF said.
Local sources have blamed the Syrian government or Russian war planes, but there has been no official comment.
Violence in Syria has intensified in recent days, despite a truce.
The upsurge in violence comes amid reports that the Syrian army, backed by Russian air power, is gearing up for a major offensive in Aleppo.
The escalation has threatened to derail the UN-brokered peace talks, which resumed last month.
On Wednesday, the UN envoy to Syria urged the US and Russia to intervene "at the highest level" to save the talks.
'Direct strike'
"MSF-supported hospital in Aleppo destroyed, at least 14 patients and staff killed, toll expected to rise,"
MSF tweeted on Thursday.
It said that the hospital was well known locally and had been hit by a direct air strike on Wednesday.
"We condemn the destruction of the al-Quds hospital, depriving people of essential healthcare," the charity added.
An activist at the scene, named as Zuhair, told the BBC that buildings around the hospital were also hit.
"It was an air strike by two rockets, heavy rockets from [a] Russian air strike," he said.
"Near the hospital one building on five floors just crumbled and just crashed down and we don't know how many dead will be under these ruins."
Syrian state news made no mention of the attack but said that rebel shelling had killed at least four civilians in government-held areas in the north of the city.
Shelling on Thursday also wounded four others, state news agency Sana reported.
Over the past week, more than 100 civilians have been killed in renewed bombardment by both rebel and government forces in Syria's largest city, according to the UK-based Syrian Observatory for Human Rights monitoring group.