A helicopter dropped containers of toxic gas overnight on a town in Syria's Idlib province, a rescue service operating in rebel-held territory said on Tuesday.
Raed Saleh, head of the Syrian Civil Defense group, told Al Jazeera that 33 civilians, including 18 women and 10 children, were brought to a local hospital after the attack.
"Just before midnight helicopters dropped five explosive barrels containing cylinders of chlorine and shards of metal on neighborhoods in Saraqeb," he said.
"We suspect it was chlorine because of the smell and the nature of the injuries - suffocation and burning, red eyes. Members of the civil defense brought them all to the nearby hospital."
The group, a network of volunteer search and rescue workers also known as the White Helmets that operates exclusively in rebel-held areas, posted a video on YouTube purportedly showing a number of men struggling to breathe and being given oxygen masks by rescue workers.
Two of the injured were in critical condition, Saleh said.
The Syrian government and its Russian allies were not immediately available for comment.
'Not the first time'
"In Saraqeb… this is not the first time that activists have reported barrels bombs being dropped that contain chlorine gas. We were told of an attack that happened last May, also in Saraqeb. And then, also, there was an attack … that happened just a month prior… close to Saraqeb," Al Jazeera’s Mohammed Jamjoom, reporting from Gaziantep along the Turkey-Syria border, said.
"This area is very close to the part of Idlib in which the Russian helicopter was downed yesterday. We’re told it’s about 15km from where the helicopter went down in rebel-held territory."
Russia's defence ministry said a Russian helicopter was shot down near Saraqeb on Monday, killing all five soldiers on board, in the biggest officially acknowledged loss of life for Russian forces since they started operations in Syria in October 2015.
The helicopter came down in Idlib province, roughly mid-way between Aleppo and the Russian air force base at Khmeimim, near the Mediterranean coast.
“Several activists are wondering if this is a retaliatory attack for the downing of the helicopter," said Jamjoom.
No group has claimed responsibility for shooting down the helicopter.
Syria's government has repeatedly been accused of dropping barrel bombs loaded with chemicals, on rebel-held areas.
Since the conflict began in 2011, there have more than 160 chemical attacks in Syria, according to the Syrian American Medical Society.
"The 161 documented chemical attacks have led to at least 1,491 deaths and 14,581 injuries from chemical exposure," SAMS said in a report published in March.
Another 133 chemical attacks were reported in addition to the 161 documented in the reported, but SAMS said the claims "could not be fully substantiated”.