Syrian state media claimed on Sunday that rebels had fired shells containing toxic gas into government-held parts of Aleppo, leaving dozens of people including civilians in need of treatment.
State news agency SANA reported that 35 people were suffering from "suffocation" after shells carrying "toxic gases" hit the frontline district of Dahiyet al-Assad and regime-held Hamdaniyeh in Aleppo.
It said people were suffering from shortness of breath, muscle spasms and numbness, but were receiving treatment.
The head of Aleppo University Hospital, Ibrahim Hadid, told state television that "36 people, including civilians and combatants, were wounded after inhaling toxic chlorine gas released by terrorists."
The allegation came on the third day of a rebel offensive to break a three-month siege of the opposition-held east of Aleppo.
Rebel groups have pledged to push from newly captured positions in the Dahiyet al-Assad district towards Hamdaniyeh.
The Syrian Observatory for Human Rights monitoring group told AFP there were "cases of suffocation among regime forces in Hamdaniyeh and Dahiyet al-Assad."
Observatory head Rami Abdel Rahman could not specify the cause.
Chlorine use as a weapon is banned under the Chemical Weapons Convention, which Syria joined in 2013 under pressure from its ally Russia.
Syria also agreed to get rid of its chemical stockpile and refrain from making any use of toxic substances in warfare.
However, there have been reports of the use chlorine gas and other chemical weapons in Syria, with the regime and the rebels both blaming one another for being responsible.
The government in Damascus argues that claims that it used chemical weapons against civilians are only meant to "serve political agendas".
A joint United Nations-Organization for the Prohibition of Chemical Weapons (OPCW) panel has concluded that government forces carried out three chlorine gas attacks on villages in 2014 and 2015.
Previous OPCW reports concluded that toxic gases were used as weapons in Syria's five-year war, but they stopped short of identifying the perpetrators.
The panel identified the 253 and 255 squadrons of the 63rd helicopter brigade, which flew from the Hama and Humaymim air bases, and the 628 squadron based in Humaymim as the perpetrators.
Russia, however, has rejected the UN inquiry as unconvincing and argued against imposing sanctions on the Syrian regime as a result.
AFP contributed to this report.