Dozens of shells were fired into the government-held city late on Saturday night, the SANA news agency reports.
People admitted to hospital reported breathing difficulties, eye inflammation and blurred vision.
Rami Abdurrahman, the head of the UK-based Syrian Observatory for Human Rights, confirmed the smell of gas filling the air after projectiles hit the al-Khalidiya neighbourhood.
It is thought to be the biggest attack on what is Syria’s second biggest city since the government clawed it back from rebel forces two years ago.
Moscow, a key ally of the Syrian leadership, backed the official claim that rebels were to blame.
But opposition officials denied using chemicals and accused president Bashar al-Assad of trying to frame them.
They claimed opposition groups did not have access to such weapons or the means to handle them.
Spokesperson Musafa Sejari said the government in Damascus was seeking to undermine an already shaky ceasefire.
The opposition also pointed out that nine people, including one child, were killed in a government attack on a village in Idlib at around the same time on Saturday, as verified by the Syrian Observatory for Human Rights.
Abdel-Salam Abdel-Razak, from the Nour el-Din al-Zinki insurgent faction, added on Twitter: “The criminal regime, under Russian instructions, is trying to accuse the rebels of using toxic substances in Aleppo. This is purely a lie.”
Syrian medical official Zaher Hajo told Associated Press that all but 15 of the 105 people who were treated at Aleppo hospitals on Saturday night have since been discharged.
He said two people in critical condition have improved.
Additional reporting by agencies
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies