The UN says the situation in Syria's city of Aleppo is catastrophic, after dozens of people were killed in attacks on targets including a hospital
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 a 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 paediatricians, MSF said.
An MSF representative, Aitor Zabalgogeazkoa, told the BBC 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 the BBC: "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.
US Secretary of State John Kerry said he was "outraged" by the hospital attack, adding: "It appears to have been a deliberate strike on a known medical facility and follows the Assad regime's appalling record of striking such facilities."
UN Secretary General Ban Ki-moon called on both the US and Russia to exert pressure to stop the violence, and demanded a credible investigation into the hospital attack.
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 upsurge in violence comes amid reports that the Syrian army, backed by Russian air power, is gearing up for a major offensive in Aleppo.
Analysis by Jim Muir, BBC News, Beirut
One of the reasons why the "cessation of hostilities" 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 Mr de Mistura 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.