VIDEO: British doctor describes aftermath of Syria gas attack

A British doctor working in Syria has said his hospital has been "overwhelmed" by victims of a gas attack in Syria's Idlib.

Speaking in a YouTube video, Shajul Islam said his patients had been hit by "some sort of chemical" far more deadly than chlorine, which Human Rights Watch says has been used by the government on several occasions in Syria.

"This is not chlorine. We do not smell chlorine on this patient," he says in the full video, which contains graphic images and should be viewed with caution. 

"This is not chlorine gas, we have had a lot of chlorine attacks. This patient has clear pinpoint pupils," he said, in reference to a symptom of nerve agents derived from organophosphates, such as sarin.

"These patients show clear signs of organophosphate attack."

At least 57 people were killed in the attack and hundreds more poisoned on Tuesday in the air attack that released "toxic gas" in Khan Sheikhun, Idlib.

Syria's government has denied it has ever used chemical weapons in the war.