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

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.  

Aleppo onslaught: Hospitals overwhelmed, few doctors left

"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.

The Syrian forces have been accused of using barrel and bunker-buster bombs with more than two dozen strikes since last night.

More than 250,000 civilians are thought to be besieged in the rebel-held sector of Aleppo, where intensive bombing by government forces and their allies has killed hundreds of people.

READ MORE: Bodies litter floor at makeshift hospital in Aleppo

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


Syrian troops launch major ground assault for Aleppo

Rights groups have accused Syrian and Russian forces of deliberately targeting medical facilities, with Amnesty International earlier this year alleging the allies were "wiping out hospitals ... [as] part of their military strategy".

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

Human Rights Watch, meanwhile, accused the Syrian government of using toxic chemicals in two recent attacks in Aleppo, and urged United Nations Security Council to take action.

Phone interviews with local residents, medical personnel, and first responders, as well as reviews of photographs and video footage, indicated that Syrian government helicopters dropped barrel bombs with toxic chemicals on two residential neighbourhoods in opposition-controlled areas on August 10 and on September 6, the group said in a statement on Wednesday.

It asked the UN Security Council to respond immediately to the reports, impose individual sanctions, and refer the situation in Syria to the International Criminal Court.

"Even after the UN confirmed the Syrian government's responsibility for chemical attacks on Syrian civilians, Damascus hasn’t stopped its criminal behaviour," said Ole Solvang, deputy emergencies director at Human Rights Watch.

"The Security Council could deter future atrocities by ensuring that the Syrian government faces real consequences for ignoring Security Council resolutions." 

Syria war: What is the importance of Aleppo?

Source: Al Jazeera News And News Agencies