Rebels have unleashed car bombs and salvos of rockets and shells to break through government lines and reach the 250,000 people besieged in the city's east.
Syrian state media today accused them of firing shells containing toxic gas into government-controlled districts.
State news agency SANA reported that 35 people were suffering from shortness of breath, numbness, and muscle spasms after "toxic gases" hit the frontline district of Dahiyet al-Assad and regime-held Hamdaniyeh.
Two days of heavy rebel bombardment have killed 41 civilians, including 16 children, and wounded 250, according to the Britain-based Syrian Observatory for Human Rights.
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The civilian toll was slammed by UN peace envoy to Syria Staffan de Mistura, whose office said he was "appalled and shocked by the high number of rockets" fired by rebels.
"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," his office said in a statement.
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.
Intense fighting today rocked western districts, battered by hundreds of rebel rockets and artillery fire, according to Observatory head Rami Abdel Rahman.
In a new toll today, the monitor said fighting had also killed 55 regime and allied fighters, as well as 64 Syrian rebels.
Fighting lasted all night and into today, with air strikes and artillery fire along the western battlefronts heard even in the eastern districts, an AFP correspondent there said.
About 1,500 rebels have massed on a 15-kilometre (10-mile) front along the western edges of Aleppo since Friday, scoring quick gains in the Dahiyet al-Assad district but struggling to push east since then.
"The advance will be from Dahiyet al-Assad towards Hamdaniyeh," said Yasser al-Youssef of the Noureddin al-Zinki rebel faction.
Hamdaniyeh is a regime-held district directly adjacent to opposition-controlled eastern neighbourhoods.
An AFP correspondent saw about a dozen civilians, including women and children, fleeing Dahiyet al-Assad on today.
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