UN “appalled” with at least 17 children among those killed, while some 1,500 rebels mass along Aleppo’s western edges.

Syrian government troops and rebels were locked in fierce fighting on Sunday on Aleppo’s western edges, where at least 41 civilians have been killed over the past three days.

The Syrian Observatory for Human Rights, a Britain-based monitor, said the death toll included at least 17 children, adding hundreds of mortars had been fired.

The northern city of Aleppo’s frontline runs through the heart of the ancient city, dividing rebels in the east from government forces in the west.

Rebels, in an opposition offensive to break a devastating siege, have unleashed car bombs and salvos of rockets and mortar shells to break through government lines.

Syrian state media on Sunday accused opposition fighters of firing shells containing toxic gas into government-controlled districts. The rebels denied the allegations.

It was impossible for Al Jazeera to independently verify the claim.

State news agency SANA reported 35 people were suffering from shortness of breath, numbness, and muscle spasms after “toxic gases” hit the frontline district of Dahiyat al-Assad and regime-held Hamdaniyeh.

United Nations Envoy for Syria Staffan de Mistura said he was “appalled and shocked by the high number of rockets indiscriminately launched” on civilian suburbs of government-held Aleppo.

“Those who argue that this is meant to relieve the siege of eastern Aleppo should be reminded that nothing justifies the use of disproportionate and indiscriminate weapons, including heavy ones, on civilian areas and it could amount to war crimes,” de Mistura said.

The head of Aleppo University Hospital, Ibrahim Hadid, told state television: “Thirty six people, including civilians and combatants, were wounded after inhaling toxic chlorine gas released by terrorists”.

Rebels deny accusations

The head of the political office of the Aleppo-based rebel group Fastaqim denied the reports. “This is a lie,” said Zakaria Malahifjim of Fastaqim.

Syria’s second city, Aleppo has been ravaged by some of the heaviest fighting of the country’s five-year war, which has killed more than 300,000 people.

In a new death toll on Sunday, the Syrian Observatory said fighting had killed at least 55 government troops and allied fighters, as well as 64 Syrian rebels.

About 1,500 rebels have massed on a 15-km front along the western edges of Aleppo since Friday, scoring quick gains in the Dahiyat al-Assad district, but struggling to push east since then.

“The advance will be from Dahiyat al-Assad towards Hamdaniyeh,” said Yasser al-Youssef of the Noureddin al-Zinki rebel faction.

Hamdaniyeh is a government-held district directly adjacent to opposition-controlled eastern neighbourhoods.

A state TV presenter, Shadi Halwi, said in a video post on his Facebook page that for the first time in government-held Aleppo, “the sound of clashes is strong, very loud.”

OPINIONS: Aleppo and the myth of Syria’s sovereignty

Chris Doyle, from the Council for Arab-British Understanding, a London-based advocacy group, said the failure to end the siege of Aleppo has unified the disparate rebel groups, with hardline fighters taking the lead.

“They have won the narrative. They’ve said, ‘look the United States, Turkey – these other countries aren’t going to help you, you have to work with us’,” he told Al Jazeera.

The situation for Aleppo residents on both sides of the frontline was bleak, he added.

“Civilians have been weaponised in this war. Both sides, but particularly the regime, have decided to use civilians as a tool, as a way of conducting the war. Instead of attacking military targets, they’ve attacked hospitals, schools. And we’re seeing now some among the opposition fighters doing the same,” said Doyle.

Russia and the Syrian government have halted air strikes on the eastern rebel-held part of Aleppo since last week to allow the evacuation of wounded civilians. But no evacuation took place and efforts to allow medical and food supplies into the besieged area also faltered.

Meanwhile, government troops kept up a ground offensive against rebel areas.

Ibrahim al-Haj – a member of the Syrian Civil Defence, or White Helmets, which operates in rebel-held Aleppo – said air strikes on Sunday on districts near the frontline caused material damage. He also said government artillery shelling killed three people and wounded seven.

‘Massive, coordinated’ assault

A government military source told AFP news agency the rebel assault was “massive and coordinated”, but insisted it was unable to break into any neighbourhoods beyond Dahiyat al-Assad.

“They’re using Grad missiles and car bombs and are supported by foreign fighters in their ranks,” he said.

Those engaged in the assault include Aleppo rebels and reinforcements from Idlib province to the west, among them Jabhat Fateh al-Sham, which changed its name from al-Nusra Front after breaking ties with al-Qaeda.

Much of the once-bustling economic hub has been reduced to rubble by artillery and air bombardment, including barrel bombs – crude unguided explosive devices that kill indiscriminately.

In late September, government troops launched an assault to recapture all of the eastern rebel-controlled territory, backed by air strikes from Russia, which began an air war in 2015 to support President Bashar al-Assad’s forces.

That onslaught spurred massive international criticism of both Moscow and Damascus.