Syria conflict: Government helicopters 'drop chlorine' on Aleppo
Volunteer emergency workers say people suffered breathing difficulties after an attack on the Sukkari area.
The reports could not be independently verified. AUN-led inquiry concluded last month the that 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 five-year civil war.
The umbrella group representing opposition factions, the High Negotiations Committee, will be hosted by UK Foreign Secretary Boris Johnson.
Foreign ministers from the Friends of Syria group of countries, which have supported the rebellion against President Bashar al-Assad, will also 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.
Source: bbc.com