Russia on Sunday said it had launched strikes on Syrian "terrorist" groups it accused of carrying out a chlorine attack the previous day.
"Air strikes were carried out by Russian air force planes," defence ministry spokesman Igor Konashenkov said in comments reported by TASS state news agency.
Russia carried out strikes after identifying the attackers' positions and finding evidence they could use such weapons again, Konashenkov said.
"As a result of the strikes, all of the rebel fighter targets were destroyed," he added.
Syrian state media and the Britain-based Syrian Observatory for Human Rights monitor reported that around 100 Syrians were hospitalised with breathing difficulties in the regime-controlled city after an attack allegedly using "toxic gas".
The monitor said air strikes then hit the edges of the last opposition Idlib bastion on Sunday, adding that they were most likely carried out by Russia.
Russia's defence spokesman said earlier that the shells fired at residential areas of Aleppo were "filled with chlorine", calling this a preliminary conclusion backed up by the victims' symptoms.
On Saturday evening, "terrorist groups" in an area of the Idlib buffer zone controlled by the Hayat Tahrir al-Sham jihadist group fired grenade launchers loaded with "explosives apparently containing chlorine", Konashenkov said.
Russian military chemists have arrived in the area to help victims and monitor the situation, Konashenkov said.
The defence spokesman said 46 people including eight children suffered from exposure to chemicals and have been hospitalised.
During the seven-year Syrian conflict, the West has accused Damascus on a number of occasions of using chemical weapons against the civilian population.
The regime of Bashar al-Assad and its top backer Moscow have consistently denied those claims, pointing the finger at rebels instead.
(With inputs from agencies.)