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.
Saudi Foreign Minister Adel al-Jubeir told the BBC that he still believed in a political solution to the conflict, and that Mr Assad was not in a “position of advantage or victory”, despite his forces’ recent advances.
“But if Bashar al-Assad continues to be obstinate and continues to drag his feet and continues to refuse to engage seriously, then obviously there will have to be a Plan B which will involve more stepped up military activity,” he said.
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.
Do you have more to share about this story – photos or video etc – you can send
them to: ([email protected])