Syrian forces continue onslaught to retake key city from rebels as medical facilities again targeted in the air war.

At least six people were killed in air strikes on two hospitals in rebel-controlled Aleppo as a Syrian government onslaught continued and the civilian death toll climbed.

UN chief Ban Ki-moon warned on Wednesday that attacks on medical facilities were war crimes.

A medical association said there were only six hospitals now remaining in the besieged city that has been been pounded by massive aerial bombardment and artillery attacks since a US-Russia implemented ceasefire collapsed last week.

“The attack happened at 4am local time [0100 GMT]. One warplane targeted both [hospitals] directly,” Adham Sahloul of the Syrian American Medical Society, which supports the hospitals, said on Wednesday.

“There are only six hospitals currently operational now that the M2 and M10 have been temporarily taken out of service,” he said.

Both hospitals had been targeted in previous aerial attacks, according to Sahloul, who described the bombings as “deliberate”.

Wednesday’s attacks were the latest in a major push by the Syrian government to recapture the key northern city that has seen the most intense bombardment of the five-year war in recent days.

Syrian troops launch major ground assault for Aleppo

Artillery shells also hit the al-Maadi neighbourhood near a bread distribution facility, killing six people, the Britain-based Syrian Observatory for Human Rights, which relies on a network of sources on the ground for its information.

It said dozens were wounded and the number of dead was likely to rise because of the severity of injuries.

It was unclear if the air strikes were executed by Syrian forces or their Russian ally, both of which are carrying out the biggest assault yet in a new campaign aimed at wiping out rebel forces and retaking a city that’s key to ending the five-year war.

Syrian forces were accused of using barrel and bunker-buster bombs with more than two dozen strikes since Tuesday night.

Al Jazeera’s diplomatic editor, James Bays, asked Syria’s UN Ambassador Bashar Jaafari if his country had bombed the two hospitals. Jaafari walked away laughing without an answer.

“It’s not clear why he was laughing considering his country is being accused of war crimes in Aleppo,” Bays said.

Syria's ambassador to the UN, Bashar Al Jaafari, laughs when asked about hospital bombings in Aleppo by @baysontheroad. pic.twitter.com/jGq9QlOwey — Al Jazeera English (@AJEnglish) September 28, 2016

More than 250,000 civilians are thought to be besieged in the rebel-held eastern sector of Aleppo, where intensive bombing by government forces and allies has killed about 400 people and wounded 1,700 others since last week.

Three employees were injured at the second hospital, among them an ambulance driver, a nurse and an accountant.

Speaking at the United Nations, Secretary-General Ban said: “This is a war against Syria’s health workers. Deliberate attacks on hospitals are war crimes. Denying people access to essential health care violates international humanitarian law.

“Even a slaughterhouse is more humane.”

Citing the Physicians for Human Rights group, Ban said 95 percent of medical workers in Aleppo before the war have been killed, detained, or fled the fighting.

Kieran Dwyer, Syria spokesman for UNICEF, said there were only 30 doctors left in eastern Aleppo for a population of 250,000 – 100,000 of those children. Medical supplies have run out.

“There are so many injured children and other civilians from these attacks that they cannot treat the most severely wounded anymore,” Dwyer told Al Jazeera. “They are … left to die.”

US Secretary of State John Kerry threatened on Wednesday to halt its joint work with Russia on Syria unless the assault on Aleppo is ended and the defunct ceasefire restored.

On Wednesday, Syrian government forces battled rebel fighters on several fronts in ground attacks in Aleppo, rebel officers said.

A senior rebel official told Reuters news agency that pro-government forces were mobilising in apparent preparation for more ground attacks in central areas of the divided city.

Another rebel officer said government forces were also attacking the rebel-held Handarat refugee camp a few kilometres to the north of Aleppo.