The Assad regime attacked a besieged Damascus residential suburb with chlorine gas on Monday, according to the pro-opposition Syrian Civil Defense (White Helmets).
At least 21 civilians, including children, were injured in the attack, which hit the city of Duma in the Eastern Ghouta region, according to civil defense.
The injured were taken to a health center in the region.
Meanwhile, regime artillery and airstrikes targeted Eastern Ghouta's cities of Harasta and Duma as well as the towns of Arbin and Marj, according to Anadolu Agency correspondents in the region.
One civilian was killed in an airstrike in Arbin, while civil defense teams rushed to the attacked areas to rescue victims stuck under debris.
Home to some 400,000 civilian residents, Eastern Ghouta has been under siege for five years, and humanitarian access to the city has been completely cut off. Hundreds are in urgent need of medical attention.
UNICEF: 30 children killed in Syria's East Ghouta this year
In the past eight months, the Bashar al-Assad regime has intensified its siege of Eastern Ghouta, making it almost impossible to bring food or medicine into the district and leaving hundreds of medical patients in need of treatment.
Notably, the district falls within a network of de-escalation zones — endorsed by Turkey, Russia and Iran — in which acts of aggression are expressly forbidden.
Syria has only just begun to emerge from a destructive civil war that began in early 2011 when the Assad regime cracked down on pro-democracy protests with unexpected ferocity.