Syrian state media said militants fired poison gas at a government-held area of Aleppo on Sunday, causing 35 people to choke, a report which a rebel official called a lie.
State news agency SANA said "shells containing poison gases" had been fired at the residential district of al-Hamdaniya.
It did not report any deaths.
The head of the political office of the Aleppo-based rebel group Fastaqim denied the report.
Rebels launched a major counter-attack against government forces in Aleppo on Friday, aiming to break a siege on the insurgent-held eastern sector of the city.
An array of rebels are taking part in the attack, including groups fighting under the Free Syrian army banner and Islamist militants.