An air strike carried out by the Syrian regime on 4 April destroyed a chemical weapons warehouse at Khan Sheikhoun in Idlib province that was reportedly used by anti-Assad rebel forces, the Russian defence ministry said.
Russian defence ministry spokesman Major-General Igor Konashenkov said in a statement on 5 April that Syrian government forces conducted an air strike on an ammunition depot containing chemical weapons.
The Russian defence ministry went on to say that rebel militants, whom it described as "terrorists", were found to have used chemical weapons in other Syrian territories.
The statement said: "Terrorists had been transporting chemical munitions from this largest arsenal to the territory of Iraq. Both international organisations and the authorities of the country had repeatedly proved their usage by terrorists.
"These chemical munitions had been also used by militants in Aleppo, their using was registered in the end of the previous year by the Russian specialists."
UK Foreign Secretary Boris Johnson responded to the Russian statement by saying: "All the evidence I have seen suggests this was the [Syrian President Bashar] Assad regime... using illegal weapons on their own people."
The statement follows the deaths of dozens of people in the town, with rebel groups and UK-based monitoring group the Syrian Observatory for Human Rights (SOHR) claiming that they were killed in a chemical weapons attack. The SOHR said that 72 people including 11 children were killed in the attack.
Countries including the US and UK accused the regime of Syrian President Bashar al-Assad of dropping chemical weapons on the town in an air strike.
Footage taken on 4 April shows victims choking and foaming at the mouth. Shortly after the alleged attack, a hospital in the town treating victims was hit by a rocket, said witnesses.
US President Donald Trump condemned what he described as "these heinous actions" by the government of Syrian President Bashar al-Assad, and blamed the "weakness" of his predecessor, Barack Obama, for the situation in Syria. Critics pointed to 2013 tweets by Trump in which he urged Obama not to become involved in the Syrian conflict.
Damascus denied using chemical weapons and said it had played no role in Tuesday's attack. It blamed rebels for the deaths, and Syrian state media said there had been an attack at a rebel "poison gas factory" in the countryside in Idlib.
The UN Security Council is set to hold discussions on the incident on Wednesday, 5 April, following requests from France and the UK for an emergency session.