The United Nations has said the situation in Syria’s city of Aleppo is outright catastrophic, after people were killed by the dozens in attacks.
Air strikes on and around the Medecins sans Frontieres-backed al-Quds hospital killed at least 27 people, while more than 30 died in other attacks.
UN envoy Jan Egeland said the next days would be vital for the humanitarian aid lifeline for much of Syria.
The violence has left a partial truce hanging by the thread.
UN envoy for Syria Staffan de Mistura warned the cessation of hostilities agreed between non-jihadist rebels and government forces on 27 February was now “barely alive”.
Separately, the Syrian government reported that 150 US troops had arrived in the town of Rmeilan in Syria’s predominantly northern Kurdish province of Hassakeh, denouncing it as an “illegitimate intervention”.
US President Barack Obama said last week he was deploying 250 troops to Syria to help certain rebel groups fight so-called Islamic State (IS).
‘Millions in danger’
Mr Egeland, the head of the UN humanitarian assistance to Syria, said he had been briefed on “the catastrophic deterioration in Aleppo over the last 24-48 hours… No-one doubts the severity of the situation.”
He warned that the humanitarian lifeline for much of the country was at risk.
“I could not in any way express how high the stakes are for the next hours and days.
“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.”
Medecins sans Frontieres (MSF) said at least 14 patients and three doctors had been killed in the air strike on al-Quds hospital.
Among those killed was Mohammed Wasim Moaz, one of the city’s last pediatricians, MSF had said.
An MSF representative, Aitor Zabalgogeazkoa, told news reporters Dr Moaz 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.”
Local sources blamed war planes from the Syrian military or from Russia, which is supporting the government of President Bashar al-Assad, for the attack.
The Syrian military denied targeting the hospital. A military source was quoted on state TV as saying: “Such news is merely an attempt to cover up terrorist crimes which target peaceful citizens in Aleppo.”
An activist at the scene, named Zuhair, told news reporters: “It was an air strike by two rockets, heavy rockets from [a] Russian air strike.
“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.”
However, Russian news agencies quoted the Russian defence ministry as saying it had carried out no air strikes in Aleppo in the past few days.
Monitoring groups said at least 20 people were killed in other attacks on rebel-held areas in Aleppo on Thursday, while at least 14 died in rocket strikes on government-controlled neighbourhoods.
The escalation in violence comes amid reports that the Syrian army, backed by Russian air power, is gearing up for a major offensive in Aleppo.