The father cradled his 9-month-old twins, Aya and Ahmed, each in an arm. He stroked their hair and choked back tears, mumbling, "Say goodbye, baby, say goodbye" to their lifeless bodies.

Abdel Hameed Alyousef lost his two children, his wife and other relatives in the suspected chemical attack in the northern Syrian town of Khan Sheikhoun that killed at least 72 people.

The 29-year-old shopkeeper had been 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.

They 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.

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It was only then, an hour later, that they began displaying symptoms.

He brought them to paramedics and, thinking they would be OK, went to look for the rest of his family. He found the bodies of two of his brothers, two nephews and a niece, as well as neighbors and friends. "I couldn't save anyone, they're all dead now," he said.

"The family was all waiting down there and were safe, but then they started choking," Mr Youssef's cousin, Alaa, told the Telegraph. "The twins suddenly began shaking and struggling to breathe. Then he watched the chemicals take hold of his wife, then his brother, nieces and nephews.

Only later was he told his children and wife had died.

"Everyone died down there in the basement, they didn't have time to get to the hospital," he said.

"All Abdulhadim kept saying to me after was 'I couldn't save any of them, brother, I couldn't save them'."

Mr Youssef hugged his children - who looked peaceful in death - one last time before laying them into the ground. Apart from a bruise on Ahmad's cheek there were no obvious signs of injuries.

"Chemical attacks leave no marks," said Dr Mamoun Najem, a doctor at al-Rahma hospital in Idlib who treated the victims. "It's a silent killer that works its way through the body slowly."

He saw dozens of patients arrive that morning and into the afternoon. He says he has never seen such severe cases of poisoning before.

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

A nurse at the hospital, who did not wish to give his name, said: "The smell reached us here in the centre; it smelled like rotten food. We've received victims of chlorine before - this was completely different.

"Victims had vomit from the nose and mouth, a dark yellow colour, sometimes turning to brown. They had paralysis of their respiratory functions - children were dying faster than adults because of this."

"Abdel Hameed is in very bad shape," his cousin Alaa said. He's being treated for exposure to the toxin. "But he's especially broken down over his massive loss."

Footage of the aftermath of the attack shows victims convulsing, struggling to breathe, and foaming from the mouth.

Idlib governorate's medical authority released an updated list of the dead on Wednesday. The youngest of the Youssef family was nine months old, the eldest was 68.

The death toll has reached 86, 30 of whom children, making it the deadliest chemical assault since 2013, when the Syrian government dropped sarin on a suburb of Damascus suburbs, killing hundreds of people as they slept.

The number was expected to rise further, however, as many are are believed to have died in their homes and their bodies yet to be taken to the hospitals to be counted.

- AP, The Daily Telegraph