In a throwback to World War I, the Syrian army is now filling barrel bombs with chlorine gas and dropping them on towns and villages in an attempt to crush a renewed opposition offensive in the civil war.
An investigation by the Daily Telegraph, comprising testimony from doctors who treated the wounded, relations of the victims and eye-witnesses, has found evidence of the regime's continued and systematic use of chemical weapons, despite having signed the Chemical Weapons Convention which bans the use of such substances.
In the northern province of Idlib, the attacks have become a near daily reality. In the past two weeks, eight separate such bombings have caused hundreds of casualties, most of them civilians.
In Mahmud's case, an explosive barrel loaded with chlorine gas landed on the kitchen of his home in the village of Talmenes in Idlib at about 10.30am on Monday.
"I saw a helicopter drop two barrel bombs on Talmenes," said Muheed, 22, who is Mahmud's uncle. "Only one of the barrels exploded but it sent a huge cloud of yellow smoke in the air ... None of them were suffering from any shrapnel wounds. But they were all in a critical condition from the gas."
Dr Abu Muhanna, who spoke using a pseudonym, also went to the field clinic to help treat the casualties. "I was driving past Talmenes when I heard the whirr of a helicopter in the sky, closely followed by a huge explosion," he said.
"I arrived minutes later. People all around me were coughing uncontrollably.
"The field hospital was overwhelmed. It's a small clinic and, in minutes, there were over 70 patients. Several dozen others were taken to hospitals in nearby towns.
"They all showed the same symptoms: itchy, swollen, red eyes, choking and burns on skin surfaces that were exposed."
Chlorine gas affects the respiratory system, irritating the throat and burning the lungs, which fill with liquid, so victims suffocate or drown.
Fifteen of the patients were transferred to hospitals that could provide ventilator support. Three of the casualties have since died and a further three remain on life-support.
Dr Jubran, another doctor present during the attack, corroborated in detail the description given by Abu Muhanna.
Film from activists and residents at Talmenes field hospital captures the pandemonium as desperate residents flood through the doors of the clinic.
President Assad agreed to a deal brokered by Russia and the United States to dispose of Syria's chemical weapons stockpile - one of the largest in the Middle East - after hundreds of people were killed in a sarin gas attack, the world's worst chemical attack in a quarter of a century, on the outskirts of Damascus last August.
The deal averted Western military intervention against the Syrian regime after the US and European allies blamed Assad's forces.
The Organisation for the Prohibition of Chemical Weapons (OPCW) has so far removed more than 80 per cent of the declared weapons stockpile - which does not include chlorine gas.
But the end-of-mission deadline tomorrow is being overshadowed by evidence that the Syrian army is once again resorting to chemical weapons.
The Foreign Office said on Thursday that it was pushing for an inquiry by the United Nations into the recent chemical attacks, and that, even if Syria met the OPCW's deadline for removing chemical stocks, it would not be certified chemical weapons free.
The US State Department, which is examining the allegations, said if the Syrian Government used chlorine with the intent to kill or harm, it would violate the Chemical Weapons Convention, which it joined as part of last September's Geneva agreement to give up its chemical weapons.
Hamish de Bretton-Gordon, a British chemical weapons expert, said he was in little doubt chlorine gas had been used in the recent attacks.
"The chlorine attacks are taking place in areas where there is heavy fighting," he said.
