The Syrian Observatory for Human Rights said there had been a suspected gas attack in Islamic State-held territory near Palmyra on Monday, amid heavy aerial bombardment of the same area which together killed at least 53 people.
Citing local sources near the site of the attack in eastern Hama province, north-west of the ancient city of Palmyra, the Observatory said there were cases of suffocation and that dozens had been wounded during heavy rocket fire on the area.
The dead included 28 children, the Observatory said.
Local sources had reported seeing dead bodies with no visible injuries, said the Observatory, a British-based group that monitors the war in Syria through a network of contacts across the country.
The reported attack came from the air and took place near the town of Uqairabat, which lies on a main road leading south into Palmyra from government-held territory, the Observatory said, without specifying who might have been responsible.
Amaq, a news service linked to Islamic State, said in an online statement that 20 people had died and around 200 were injured from breathing problems “as a result of a Russian air attack with sarin gas”.
Ahmad al-Dbis, of the Union of Medical Care and Relief Organizations (UOSSM), said about 86 people had died, and about 250 were injured in the attacks on Monday.
UOSSM is a coalition of international aid agencies which funds hospitals in Syria. It used to work with staff in the area of the attack before Islamic State took over.
Both the Syrian army and Russia have denied using chemical weapons.
UN investigators established that sarin gas was used in parts of the rebel-held Ghouta suburbs of the Syrian capital Damascus in 2013. The United States accused Syria’s government of conducting the attack, which it estimated killed around 1,400 people, but Damascus denied responsibility and blamed rebels.
A United Nations investigation this year found that the Syrian military had used chlorine in attacks on rebels and that Islamic State had also used chemical weapons in attacks.
Damascus has said that the conclusions of that investigation were wrong.
On Sunday, Islamic State militants recaptured Palmyra despite dozens of Russian air strikes attempting to push them back. It had previously been driven from the city in March.