A video posted to YouTube apparently showed several men struggling to breathe in the aftermath.
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Syria Civil Defence workers, who went to the scene of the attack, said they suspected it was chlorine but could not verify that.
“Medium-sized barrels fell containing toxic gasses”, a spokesman for the group said.
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”.
Assad’s government and its Russian allies declared a joint humanitarian operation for the besieged area on Thursday, bombarding it with leaflets telling fighters to surrender and civilians to leave.
The government of embattled Syrian President Bashar al-Assad has repeatedly been accused of attacking rebels with crude chemical bombs.
But government troops backed by Russia’s air force have put up a fierce defence, the monitor said.
Rudskoi said 324 civilians so far have left the militant-controlled part of Aleppo through corridors set by the Syrian government last Thursday.
A Russian assault helicopter nicknamed the ” Terminator” was shot down Monday by Syrian rebels, near a town where civilians reported toxic gas dropped on them in possible retaliation.
The Syrian National Council, a coalition of opposition groups, blamed the Assad regime for the alleged attack in Idlib province, where a Russian helicopter had earlier been downed after making what the Kremlin said was an aid delivery to Aleppo.
An alliance of forces, including hardline jihadist factions, is the dominant power in Idlib, but it was not clear which group brought the helicopter down.
Meanwhile clashes are continuing near Aleppo, where rebels have launched an offensive to break a government siege.
According to the United Nations, 300,000 people are trapped in eastern Aleppo and food supplies could run out in a few weeks.
The Britain-based Syrian Observatory said at least 11 people, including five children, were killed when bombs were dropped in a market in rebel-held Atareb, west of the city of Aleppo.
The strikes came despite an appeal by US Secretary of State John Kerry on Monday for Russian Federation to “restrain” itself and its ally in Damascus from “offensive operations”.
The Russian Defense Ministry also said that an aircraft of the Abakan Air, Russian aviation company, delivered 21 metric tons of humanitarian aid to Deir ez-Zor.
A photo taken on 1 August, 2016 reportedly shows Syrian rebels gathered round the wreckage of the downed helicopter.
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Rebels mainly from Jaish al Fatah alliance of Islamist groups pushed from the south of the city while fighters from Free Syrian Army (FSA) brigades pounded government-held areas of Syria’s most populous city and commercial hub before the war broke out in 2011.
