The U.S.-led coalition against Islamic State on Thursday denied a Syrian army report it had carried out an air strike that had hit poison gas supplies belonging to Daish and caused the deaths of hundreds of people.

A statement by the Syrian army, circulated by state media, said the alleged incident late on Wednesday in the eastern Deir al-Zor province proved that Daish and an al Qaeda-linked militants "possess chemical weapons".

Syria and its ally Russia have both asserted that insurgents fighting the government of President Bashar al-Assad have stocks of banned poison gas, seeking to pin the blame on the rebels for a chemical weapons attack that killed scores of people in northwest Syria on April 4.

Washington and its allies say there is no doubt that the Syrian military carried out that attack, which prompted U.S. President Donald Trump to order cruise missile strikes on a Syrian air base last week.

Responding to Thursday's Syrian army claim, U.S. Air Force Colonel John Dorrian, a spokesman for the coalition, said it had carried out no air strikes in the area of Deir al-Zor at the time.

"The Syrian claim is incorrect and likely intentional misinformation," Dorrian said in an email to Reuters.

The Russian defence ministry said it had no information about people killed in an attack by international coalition forces in Deir al-Zor, according to RIA news agency. A ministry spokesman said Russian forces had sent drones to check the area.

Syrian President Bashar al-Assad told news agency AFP in an interview published on Thursday that the alleged April 4 poison gas attack in Idlib province was a "100 percent fabrication".

The British delegation at the world's chemical weapons watchdog said samples taken from the alleged attack site tested positive for the nerve agent sarin.

"We believe it is highly likely that the attack was carried out by the Assad regime," British Prime Minister Theresa May said in a televised statement. "Apart from anything else, we believe it's only the regime that has the capability to make such an attack."

The retaliatory U.S. strike on the Syrian air base was the first time Washington has deliberately and directly targeted the Syrian government. The United States is separately waging an air campaign against Daish in eastern Syria.