The Syrian army said that an air strike late on Wednesday by the U.S.-led coalition hit poison gas supplies belonging to Islamic State, releasing a toxic substance that killed "hundreds including many civilians."
The incident in the eastern Deir al-Zor province proved that Islamic State and al Qaeda-linked militants "possess chemical weapons", a statement by the army flashed on Thursday by Syrian state TV said.
The report could not immediately be independently verified.
The United States launched cruise missiles at a Syrian air base last week, in response to a deadly poison gas attack in the west of the country that Washington blamed on President Bashar al-Assad's government.
Syria and its ally Russia deny Damascus carried out any such chemical attack. Moscow has said the poison gas in that incident last week in Idlib province belonged to rebels.
The U.S. strike on the Syrian air base was the first time Washington has deliberately and directly targeted the Syrian government. It is separately waging an air campaign against Islamic State in eastern Syria.