The crude unguided explosive devices used by government forces have been blamed for thousands of civilian deaths.
At least two barrel bombs have hit the largest hospital in the rebel-held eastern side of Syria’s Aleppo city, according to the medical organisation that supports it.
The facility, known as M10, had already been hit by heavy bombardment on Wednesday along with the second-largest hospital in the area in what Ban Ki-moon, the UN secretary-general, denounced as “war crimes”.
Referring to Saturday’s attack, Adham Sahloul, of the Syrian American Medical Society, said: “Two barrel bombs hit the M10 hospital and there were reports of a cluster bomb as well.”
Barrel bombs are crude, unguided weapons that kill indiscriminately.
— sams_usa (@sams_usa) September 30, 2016
Al Jazeera’s Charles Stratford, reporting from Gaziantep on the Turkish side of the Turkish-Syrian border, said only half of the M10 hospital was operational before Saturday as a result of previous attacks.
“We now understand it is now being put out of action completely,” he said.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said one person was killed in the latest attack, but could not immediately confirm if the victim was a patient at the facility or a member of staff.
Stratford said there were reports of at least three deaths.
“We spoke to a member of the White Helmets, they said there was around 60 people inside the hospital being treated at the time,” he said.
“They say that those people have now been evacuated to a smaller clinic in this area.”
Al Jazeera’s Amr al-Halabi, who assessed the damage on Saturday, said the bodies of the victims could be seen on the hospital grounds.
“They were killed as a result of the raids,” he said. “They were patients here before the strike.”
“One of the rockets dropped just in front of the main entrance of the hospital near the emergency entrance. The hospital has suffered severe damage.”
One week into a new Russian-backed Syrian government push to capture rebel-held areas in eastern Aleppo, residents said that the city was being subjected daily to ferocious bombardment.
At least 30 civilians were killed by Syrian government or Russian air strikes overnight in Aleppo, activists told Al Jazeera on Saturday.
A further 18 were killed in air strikes on various towns in Damascus’s eastern countryside, activists said.
At least 320 people, including nearly 100 children, have been killed in Aleppo since a US-Russian brokered ceasefire collapsed on September 19, according to UNICEF.
Fighting has also intensified in other parts of Syria.
Civilians have been killed by shelling and air strikes in rebel-held Idlib province.
The Syrian army has launched a counteroffensive in Hama after losing territory to the rebels.
And activists say strikes have increased on residential areas of Rastan, in Homs province.
Against this backdrop of rising civilian casualties, John Kerry and Sergey Lavrov, the top US and Russian diplomats, spoke by telephone for a third straight day on Friday.
Lavrov said Russia was ready to consider more ways to “normalise” the situation in Aleppo.
However, he criticised what he called the US failure to separate moderate rebel groups from those Russia calls “terrorists”.
He argued that this allowed forces led by Jabhat Fateh al-Sham, formerly known as al-Nusra Front, to violate the US-Russian ceasefire agreed on September 9.
Russia and the US have traded blame for the collapse of the truce, with Kerry admitting on Thursday that months of diplomacy to end the war had reached a stalemate.
On Friday, however, the US said that it would not, for the time being, carry through on the threat it made earlier in the week to halt diplomacy if Russia did not take immediate steps to halt the violence.
Nearly 300,000 people – including 100,000 children – are trapped in Aleppo’s eastern districts, which civil defence workers say have been hit by 1,900 bombs in the past week.
The air campaign has wreaked destruction on hospitals, clinics, residential buildings, water stations and electric generators.
On Friday, Russia’s Izvestia newspaper reported that a group of Su-24 and Su-34 warplanes had arrived at Syria’s Hmeymim base.
The Su-25 is an armoured twin-engine jet that was battle-tested in the 1980s during the Soviet war in Afghanistan.
It can be used to strafe targets on the ground, or as a bomber.
Russia’s defence ministry did not immediately respond to a request for comment.
The US state and defence departments declined comment on the Izvestia report.
The newspaper report came hours after the Syrian observatory reported that 9,364 people, including about 3,800 civilians, have been killed in one year of Russian air raids in Syria.
Later on Friday, the Russian government rejected the figures.
An audio recording obtained by the New York Times on Friday suggested that Kerry was frustrated that his diplomatic efforts to end the conflict had not been backed by a serious threat of US military force.
“I think you’re looking at three people, four people in the administration who have all argued for use of force, and I lost the argument,” Kerry told a group of Syrians, diplomats and others o n the sidelines of the UN General Assembly meeting in New York last week, according to the 40-minute recording of the session.
“We’re trying to pursue the diplomacy, and I understand it’s frustrating. You have nobody more frustrated than we are.”.
The recording was made by a non-Syrian who attended the session, the newspaper reported, adding that several other participants confirmed its authenticity.
According to the New York Times, several people in the meeting pressed Kerry on what they saw as contradictions in US policy.
It said one activist, Marcell Shehwaro, asked “how many Syrians” had to be killed to prompt serious action.
Kerry responded that “Assad’s indifference to anything” could push the US administration to consider new options, according to the newspaper, but he also said that “any further American effort to arm rebels or join the fight could backfire”.
Al Jazeera’s John Hendren, reporting from Washington DC, said: “Kerry was speaking in frank and frankly undiplomatic terms.
“These comments show his own frustration with US policy, US Congress and the American failure to follow up on President Obama’s threat that if Syria crossed a certain red line with chemical weapons … the US would act.
“Now there is evidence of chlorine gas being used in barrel bombs there, yet US policy has not changed.”