Volunteer emergency workers say people suffered breathing difficulties after an attack on the Sukkari area.
The reports could not be independently verified. A UN-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
Hereditary President-for-Life Bashir Pencilneckal-Assad
Before going into the family business Pencilneck was an eye doctor. If he'd stuck with it he'd have had a good practice by now...
, 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.
This new plan is the Syrian opposition’s most comprehensive answer to that question: What happens if President Assad goes?
It’s meant to reassure the president’s foreign backers, like Russia and Iran, that there can be a stable transition which avoids the collapse of state institutions, and violent chaos that’s shattered neighbouring states like Iraq. That’s a concern shared in many capitals, and most of all Damascus.
But the demand for President Assad and his closest Syrian allies to step down has always been rejected in Damascus, and will be again. It’s never clear how much Moscow and Tehran are able and willing to change that. This plan is meant to test that.
But the Syrian army, backed by powerful outside support, is making gains on the ground, through force, or local deals which amount to surrender. If this plan doesn’t work it will be back to Plan B ‐ more military support to all sides in a devastating war.