WATCH: 'I saw my family die in agony'

Abdulhamid al-Youssef sobbed as he carried his infant twins, wrapped in matching white shrouds, to their last resting place.

Nine-month-olds Ahmad and Aya died on Tuesday in a chemical attack on the town of Khan Sheikhoun in Idlib, northern Syria, as did Youssef's wife Dalal and 16 other members of his family.

Yesterday he buried them all in an unmarked mass grave.

Youssef, a supermarket cashier, was at work when the air strike hit close to his home just after 6.30am. When his wife called to tell him what had happened, he rushed home to be with them.

His family appeared to be fine, but as a precaution he took them all down to the basement of a nearby building in case of another strike. It was only an hour later that they began displaying symptoms.

"The family was safe down there. But then they started choking," said Youssef's cousin, Alaa.

"The twins began shaking and struggling to breathe."

He then watched as the chemicals took hold of his cousin's wife, brother, nieces and nephews.

"Everyone died down there in the basement. There wasn't even time to get them to the hospital."

Youssef hugged both children, one last time before laying them in the ground. Apart from a bruise on Ahmad's cheek there were no obvious signs of injuries.

They looked peaceful in death.

"Chemical attacks leave no marks," said Mamoun Najem, a doctor at al-Rahma hospital in Idlib who treated the victims.

"It's a silent killer that works its way slowly through the body."

He said he had never seen such severe poisoning cases before.

"Their pupils were pinpricks, their skin, cold. They were unresponsive, like zombies," he said.

"The smell reached us here in the centre; it smelled like rotten food. We've had victims of chlorine before - this was very different," said a nurse at the hospital.

"Victims had vomit coming from their noses and mouth - a dark yellow, sometimes brown, colour.

"There was paralysis of the respiratory functions; that's why children died faster than adults."

Western intelligence agencies hope to conduct biological tests on survivors to match findings against specimens of the sarin taken from the Syrian military four years ago. There is suspicion the government never declared some of its sarin stockpile to UN inspectors supervising the surrender of supplies.

The surrender was made in terms of a deal, brokered with the help of President Bashar al-Assad's Russian allies, to avert military action by the US after a chemical attack on Damascus suburbs in 2013 killed up to 1300 people.

Footage of the aftermath of Tuesday's attack, showing victims convulsing, struggling to breathe and foaming from the mouth, has shocked and outraged the world.

The death toll reached 86 yesterday - among those, 30 children - making it the deadliest chemical assault since 2013.

Neither side has denied there was a chemical attack but Russia has claimed a Syrian government air strike hit a rebel chemical weapons warehouse. The US and Britain put the blame on President Bashar al-Assad's regime.

Hasan Haj Ali, a senior rebel officer, called the Russian assertion "a lie", saying the rebels did not have the capability to produce chemical weapons and no military positions in the area were bombed.

Assad's regime insists it had no reason to attack Khan Sheikoun, 11km from the nearest frontline. Its opponents say the same.

Chemical warfare experts agreed that the rebels had neither the money nor the materials to make the gas - and they would not have been able to without being noticed. Unlike the regime, the rebels have not used sarin gas to date.

"There are no warehouses, no factories in Khan Sheikoun," said Youssef. "Not even fighters - just civilians trying to survive. The opposition would never bomb its own people."

©The Daily Telegraph