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 blamed Syrian or Russian war planes. The Syrian military has denied targeting the hospital.
Monitors say attacks by both sides left 34 dead and dozens wounded on Thursday.
Violence in Syria, and particularly in Aleppo, has intensified in recent days, despite a partial truce.
The International Committee of the Red Cross says Aleppo is on the brink of a humanitarian disaster.
The upsurge in violence comes amid reports that the Syrian army, backed by Russian air power, is gearing up for a major offensive on the city.
The escalation has threatened to derail UN-brokered peace talks in Geneva, 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.
"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 said.
An activist at the scene, named 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."
The Syrian military denied targeting the hospital.
An MSF representative, Aitor Zabalgogeazkoa, told the BBC that the paediatrician who died was Mohammed Wasim Moaz, who had worked at the hospital since 2013.
Mr Zabalgogeazkoa said: "He kept it going, was always there and always worried about the needs of the people. He was honest and very committed. He worked in conditions you cannot even begin to imagine."
Rami Abdurahman, head of the monitoring group the Syrian Observatory for Human Rights, told the BBC that Dr Moaz, 36, was the last paediatrician in the rebel-held part of Aleppo and another was to be sent on Thursday to take his place.
The Observatory said rebel rocket fire on government-held areas on Thursday had killed 14 civilians while attacks by pro-government forces on rebel neighbourhoods had killed at least 20 people.
Analysis by Jim Muir, BBC News, Beirut
The two-month-old "cessation of hostilities", which brought at least a relative lull to some parts of Syria, is indeed "hanging by a thread" as the UN mediator Staffan de Mistura put it.
One of the reasons why it is now at death's door was reflected in the fact that from the outset it was not called a ceasefire or even a truce, because several factions were excluded, including not just the Islamic State militants but also the al-Qaeda-linked Nusra Front.
Nusra fighters are present in almost all combat zones, and are mixed up with other groups such as Ahrar al-Sham that Russia is now pressing to have added to the international terror list.
That has meant that hostilities have continued and intensified in many areas, with the government able to claim its attacks are legitimate.
Now state forces are reported to be building up in Aleppo as violence escalates there, raising fears that a long and costly all-out battle for the contested city may be looming.
That would put paid both to the lull and to the Geneva peace talks, prompting the UN envoy to urge the US, Russians and others to press their clients on the ground to ease off, so that stalled negotiations have a chance of resuming.
Syrian state news made no mention of the hospital attack but also said that rebel shelling had killed at least 14 civilians in government-held areas in the north of the city.
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 Observatory.
Speaking on Wednesday after briefing the UN Security Council on the peace process, the envoy to Syria, Staffan de Mistura, warned that the fragile cessation of hostilities agreed in February could collapse "at any time".
For the peace talks in Geneva to succeed, he added, hostilities would need to be reduced to the levels immediately following the February agreement.
Calling on the US and Russia to co-operate, Mr de Mistura said the legacies of both President Barack Obama and President Vladimir Putin were linked to the success of the peace process in Syria.
Although some fighting has continued since the partial truce started on 27 February, the numbers of deaths overall have fallen.
In the eight weeks up to and including 26 February, a weekly average of 239 civilian deaths was recorded by the Violations Documentation Center in Syria.
In the eight complete weeks from 27 February, the average was 120.
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.