The main trauma hospital in the rebel-held east of the Syrian city of Aleppo has been hit in an air strike for the third time in a week, activists say.
The extent of the damage was unclear, but video purportedly of the aftermath showed damaged walls and craters on one side of the M10 hospital.
Three maintenance workers were among those killed, a medical charity said.
Hundreds have died since government forces launched an offensive to take full control of Aleppo two weeks ago.
Earlier, a senior United Nations official warned that the healthcare system in eastern Aleppo had been "obliterated" and called on both sides to agree 48-hour humanitarian pauses to allow in aid.
Adham Sahloul of the Syrian American Medical Society, which supports the M10 hospital, reported that three maintenance staff had been killed in Monday's attack.
One bunker-busting bomb had left a 10m-deep (33ft) crater outside the front entrance, and there were fears the rest of the building might collapse, he added.
"The hospital is now not usable at all. It is not salvageable," he was quoted as saying by the AFP news agency.
The Syrian Observatory for Human Rights, a UK-based monitoring group, put the death toll at six, while the opposition Local Co-ordination Committees network reported seven deaths in the area.
Meanwhile, the US state department confirmed it was suspending negotiations with Russia after it said Moscow had increased bombardments of civilian targets.
Russia and the US were due to convene in Geneva to try to co-ordinate air strikes against jihadist groups, but American officials were told to return home.
Spokesman John Kirby said in a statement the decision was "not taken lightly", but that both the Russian and Syrian governments had chosen a military course of action "inconsistent with the Cessation of Hostilities".
He also said Russia had "failed to live up to its own commitments - including its obligations under international humanitarian law".
UN Emergency Relief Co-ordinator Stephen O'Brien has warned that medical facilities are being destroyed "one-by-one" as eastern Aleppo is pounded with bunker-busting bombs, barrel bombs, mortar rounds and artillery shells.
Patients are being turned away, no medicines are available to treat even the most common ailments, and the number of people requiring urgent medical evacuations is likely to rise dramatically with clean water and food in very short supply.
"We have a besieged community of about 275,000 people, who are living in total fear of further air strikes, and being oppressed to a point where they don't know what is going to happen next," Mr O'Brien told the BBC on Monday.
People were also only able to eat small amounts of food once a day, as supplies dwindled and prices rocketed after four weeks of siege, he said.
"It is not that they are at the edge - they are now falling into the terrible abyss," he added. "It is pitiless, it is merciless, and we do need now to get the guns to fall silent so we can get the humanitarian supplies in."
Mr O'Brien said aid was ready for delivery, but weekly 48-hour humanitarian pauses were needed "at the very least" to ensure safe passage into and out of the city.
The UN was unable to reach eastern Aleppo during a cessation of hostilities brokered by the US and Russia at the start of September.
A failure to obtain the necessary permits from the Syrian government and a deadly attack on a Syrian Arab Red Crescent aid convoy outside Aleppo prevented deliveries before the truce collapsed after only a week.