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Activists and emergency responders claimed Tuesday that dozens of people were injured after a helicopter dropped canisters of toxic gas on rebel-held territory in northern Syria.

The Syrian Civil Defence group said the attack — which has not been independently verified by NBC News — was carried out in the northern city of Saraqeb on Monday night, near to where a Russian helicopter was shot down earlier in the day.

A still image taken on Tuesday from a video said to be taken on Monday and posted on social media shows two men standing over a man on a bed, making him sit up as he breathes through a mask in what is said to be in Saraqeb, Syria. Handout / Reuters TV

A spokesman for the organization said in an emailed statement to NBC News that 33 people — including 18 women and 10 children — were taken to hospital showing "signs of severe respiratory distress." Three of the victims were in a "critical condition," he added.

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The group, also known as the "White Helmets," released video showing what it said were several victims lying on what appeared to be rudimentary hospital beds, breathing through masks and hooked up to IV drips.

One person helping the purported victims was wearing a jacket bearing the Syrian Civil Defence logo.

According to the group, a helicopter dropped two barrels, each containing five canisters full of chlorine as well as "iron balls of various sizes."

After taking the victims to hospitals, the volunteers removed what was left of the barrels and canisters, according to the organization, which describes itself as a neutral search-and-rescue operation.

The U.K.-based Syrian Observatory for Human Rights also reported a helicopter attack on Saraqeb using "explosive barrels." It said that "activists accused regime forces of using explosive barrels which contained gasses."

Fouad Basbous, a photographer in Saraqeb, told NBC News he saw clouds of what looked like smoke or gas rise over several areas of the city just before midnight on Monday.

Western powers including the U.S. have accused Syrian President Bashar Assad of using chemical weapons during his regime's five-year civil war — a charge he has denied. Rebel groups have also denied accusations by Assad and his Russian backers that they have used chemical weapons.

In 2013, the United Nations and the Syrian government agreed to destroy the state's declared stockpile of chemical weapons, a process completed in January 2016, according to Reuters.

The wreckage of a Russian helicopter shot down in the north of Syria's rebel-held Idlib province on Monday. AMMAR ABDULLAH / Reuters

Monday's alleged attack came just hours after a Russian helicopter was shot down between the besieged city of Aleppo and Russia's Hmeymim airbase. Moscow said it was returning from delivering humanitarian aid. No group has claimed responsibility for downing the helicopter.

Dmitry Peskov, spokesman for Assad ally Vladimir Putin, told Russian state-run news agency TASS that he didn’t “have any information” when asked about the alleged attack.