Russia says it has carried out air strikes against Syrian rebels it accuses of wounding more than a hundred people in the city of Aleppo with a chemical attack.
The Syrian government also blamed the rebels, showing images on state media of Aleppo residents being treated in hospital as they struggled to breathe.
The rebels denied the allegations, saying Moscow and Damascus are using the claims as a pretext for an attack on opposition-held areas.
There had been no major offensive in northern Syria since September, when Russia and Turkey, which backs the rebels, agreed to create a buffer zone to separate government forces from rebel fighters in Idlib province.