According to the activists, government forces dropped chlorine-filled barrel bombs.
Syrian activists say chemical gas was used in a district in Aleppo and put out video purporting to show children and adults being treated for breathing difficulties.
The chemical gas was allegedly used in the al Qasiliah neighbourhood, with nearby Bustan al Qasr also reportedly shelled.
Syrian government forces have recently launched attacks against Bustan al Qasr, one of the largest districts still to be held by revels.
According to the activists, government forces dropped chlorine-filled barrel bombs. The footage shows children in distress, some seemingly wearing oxygen masks.
The footage and the allegations cannot be independently verified.
The Syrian government has dropped chlorine bombs in at least three attacks over the past two years, according to a special joint investigation of the United Nations and an international chemical weapons monitor.
The use of chlorine as a weapon is banned under the 1997 Chemical Weapons Convention, which Syria joined in 2013.
Syrian President Bashar al Assad is on the verge of recapturing all of Aleppo from rebels.
About 93% of the city is now controlled by the Syrian government, Russia said, while the insurgents are holed out in a handful of areas mostly south of the historic Old City.
The city, once Syria's largest, has been divided since 2012 and has been a key battleground in the country's civil war, as human rights groups have warned of a humanitarian disaster.
On Saturday, Russian warplanes and Syrian artillery bombarded rebel-held districts, and rebels responded with shelling of government-controlled areas.
The Syrian Observatory for Human Rights monitoring group said several people were killed in the rebel shelling on Saturday. Hundreds have been killed in recent weeks, mostly in government bombardments, it says.
As the fighting raged on the ground, diplomatic efforts were under way in Europe.
US Secretary of State John Kerry, Foreign Secretary Boris Johnson and European and Arab diplomats are meeting members of Syria's opposition in Paris on Saturday.
Meanwhile, US and Russian military experts and diplomats are meeting in Geneva to work out a way to save lives, but an agreement looked elusive.
In Paris, Western powers called for Russia and Syria to "let people leave Aleppo".
"Russia and Assad have a moment where they are in a dominant position to show a little grace," said Mr Kerry.
He added: "The indiscriminate bombing by the regime violates rules of law, or in many cases, crimes against humanity, and war crimes."
Germany said Syrian opposition backers were seeking a political solution, but there was no agreement on reaching a truce.
The war has killed more than 300,000 people and made more than 11 million homeless.