Syrian government forces have been accused of dropping barrel bombs containing chlorine from helicopters on a suburb of Aleppo, injuring 80 people.
Volunteer emergency workers say people suffered breathing difficulties after an attack on the Sukari area.
The reports could not be independently verified. A UN-led investigation in August found the government had used chlorine on at least two occasions.
The Syrian government has always denied using chemical weapons.
It comes as Syrian opposition leaders prepare to meet in London on Wednesday to launch a new plan for a political transition to try to end the conflict.
The opposition’s main political group, the High Negotiations Committee, will be hosted by UK Foreign Minister Boris Johnson and foreign ministers from the Friends of Syria group will attend.
Ibrahem Alhaj, a Syrian Civil Defence rescue worker, said he had reached the scene of the Aleppo attack shortly after a helicopter dropped barrels containing what he said were four chlorine cylinders on Tuesday.
Syrian Civil Defence, a volunteer emergency response team that operates in opposition-held areas, posted video on its Facebook page showing distressed children using oxygen masks to breathe.
The group accused the government of another chlorine attack in August.
Russia, an ally of the Syrian government, has accused rebels of firing shells containing “toxic gas” at government-held areas in Aleppo.
Chlorine is a common industrial chemical, but its use in weapons is banned by the Chemical Weapons Convention.
The UN said on Tuesday that a brief period of relief early this year for civilians caught up in the war in Syria had been replaced by an even more brutal resumption in fighting.
In its 12th report on Syria, it said the cessation of hostilities in February had allowed some towns to receive their first aid in years but it only lasted a few weeks.
The report says 600,000 Syrians now live under siege with a further 300,000 trapped in the city of Aleppo.
On Sunday, Syrian government forces were reported to have recaptured parts of Aleppo city which were lost to rebels last month, placing rebel-held districts in the city’s east once again under siege.
Monitors said government troops had recaptured two military academy sites in the Ramouseh district south of the city and severed a recently established rebel supply line.
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Source: BBC