Russia has carried out air strikes against Syrian rebels it accuses of launching a chemical attack on the government-held city of Aleppo.
Both Syria and its Russian allies say shells carrying toxic gas injured about 100 people late on Saturday.
State media showed images of Aleppo residents being treated in hospital as they struggled to breathe.
The rebels deny carrying out a chemical strike and say the claims are a pretext for an attack on opposition-held areas.
Parts of the Aleppo region, as well as the neighbouring provinces of Hama and Idlib, are controlled by Turkish-backed rebels and jihadists.
On Sunday the Russian defence ministry said warplanes had launched strikes on "the identified positions of terrorist artillery in the area that shelled civilians in the city of Aleppo" the previous evening.
It added that all "militant" targets had been "destroyed". There is no word on any casualties.
Earlier, the Syrian Observatory for Human Rights, a UK-based group that monitors the war via a network of activists on the ground, said about 100 people, including women and children, had been treated for breathing difficulties following a strike on western parts of Aleppo on Saturday.
The Damascus government and Russia say rebels used chlorine gas in the attack.
Zaher Batal, the head of the Aleppo Doctors Syndicate, told Reuters news agency that it was the first gas attack against civilians in the city since the start of the conflict in 2011.
But Abdel-Salam Abdel-Razak, an official from the Nour al-Din al-Zinki, an influential jihadist group in the area, dismissed the reports as "purely a lie", Reuters news agency reported.
Western countries and the UN have accused government forces of using chemical and nerve agents on numerous occasions since the start of the civil war.
There has been no major offensive in northern Syria since September, when Russia and Turkey agreed to create a buffer zone to separate government forces from rebel fighters in Idlib province.