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 34 people.
Citing local sources near the site of the attack in eastern Hama province, northwest 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.
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”.
Both the Syrian army and Russia have denied using chemical weapons. 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.