Pope Francis urged all sides to protect civilians. Photo: BBC NEWS

Speaking at a public audience in St Peter's Square in Rome, he called it "an already martyred city, where everybody is dying".

Russian-backed Syrian government forces have launched a fierce campaign to take control of rebel-held eastern areas.

Air strikes continued to hit the besieged districts overnight.

Pope Francis urged all sides to "commit themselves with all their strength to protect civilians".

"This is an imperative and urgent obligation. I appeal to the consciences of those responsible for the bombings, who will one day will have to account to God," he said.

Reports citing medical workers say that two major hospitals in rebel-held eastern Aleppo were put out of service by air raids overnight.

But one activist in the city tells the BBC that the hospitals still appear to be operational.

In the past few days, pro-government forces have begun ground operations to try to recapture more territory from rebel forces. They have made advances in the city centre, reports say.

The government of President Bashar al-Assad and its Russian backers have been accused of war crimes and "barbarism" by the US and its Western allies, who back the rebels.

Moscow has vigorously denied the allegations and criticised their "unacceptable" rhetoric.

Separately, fresh allegations of chemical weapons use on the part of Syrian government have been made by the New York-based group Human Rights Watch.

It said that Syrian government helicopters appeared to have used chemical weapons in two incidents in Aleppo on 10 August and 6 September that killed five people, including two children.

"After each attack, dozens of people suffering from a shortage of breath, coughing, reddened skin and eyes, and excessive tearing sought medical treatment in hospitals," the group said.

Aleppo and surrounding area map

Syrian rebels have been supplied with a new type of surface-to-surface Grad rocket by their foreign backers to help defend against the current offensive, a rebel commander told the Reuters news agency.

The rockets, supplied "in excellent quantities", will be used in Aleppo, Hama and the Mediterranean coastal region, Fares al-Baoush said.

It was not clear which foreign states supplied the rockets.

Aleppo, once Syria's largest city and the country's commercial and industrial hub, has been divided roughly in two since 2012, with President Bashar al-Assad's forces controlling the west and rebel factions the east.

In the past year, government troops have gradually broken the deadlock with the help of Iranian-backed militias and Russian air strikes. Earlier this month, they severed the rebels' last route into the east and placed its 250,000 residents under siege.

Children in Aleppo have made up a large proportion of casualties from air strikes, according to aid groups. At least 100,000 children remain trapped in the eastern part of the city.

In the government-held west, 49 children were killed by rebel shelling in July alone, the New York Times reports, citing the Syrian Observatory for Human Rights.

On Tuesday, the US pledged to provide an extra $364m (£276m) in humanitarian aid to people affected by the Syrian war.

The World Health Organization meanwhile called for the "immediate establishment of humanitarian routes" into Aleppo, where hospitals are running out of supplies and rubble-strewn streets are preventing ambulances from getting through.