“Eighty-six people were killed in the attack, which is not a lot for sarin. If you look at Halabja (the 1988 chemical attack carried out by Saddam Hussein’s forces against the Kurdish city) we think just five tonnes of sarin was used and more than 12,000 people died,” he said.
“Sarin degrades fairly quickly and becomes less toxic over time, so we could be looking at an attack using old sarin.”
Brig Gen Sakat believes the regime has also experimented with mixing different gases – like sarin and tear gas – in order to create a mélange of symptoms that would make the cause hard to identify.
The Syrian government has repeatedly denied bombing Khan Sheikhoun with chemicals, saying its air strikes hit a warehouse the opposition had been using to store toxic materials.
However, British investigators with the OPCW reported on Thursday that samples from victims tested positive for sarin, a nerve agent the rebels are not known to possess.
Brig Gen Sakat said it was not an accident the town was chosen for such an attack and that the regime’s use of chemicals is “always strategic”.
The northern rebel-held town links other opposition areas around Homs, Hama, and most importantly Idlib, the largest urban area under their control. “If you can take Khan Sheikhoun, or force its residents to surrender, you can take the road that connects them,” he said. And chemicals can terrify people into surrendering, he added.
He said Assad became more brazen after the US failed to act when he crossed Barack Obama’s “red line”.
“He experimented and realised everyone was silent to all his crimes: the barrel bombing of civilians and even chemical ones,” Brig Gen Sakat said. “He began acting in the face of the UN and the international community.”
He confirmed that only the head of the army, namely Assad, has the authority to order nerve gas attacks because of the potential fallout.
However, those involving chlorine and other less-deadly chemicals can be signed off by senior local commanders trusted by the regime, usually those from the same Alawite sect as the president.
In the months before he defected, Brig Gen Sakat said he was personally ordered by his commander, Gen Ali Hassan Amar, to carry out three chemical attacks.
They took place in October 2012 on the southern town of Sheikh Maskeen, in December 2012 on nearby Harak, and in January 2013 on Busra al-Harir – all places where demonstrations had been taking place against Assad.
He was told the people “needed to be reorientated”, but Brig Gen Sakat knew the intended target was civilians supportive of the rebellion.
He was told to prepare phosgene, which at high concentrations damages the lungs within seconds and causes death by suffocation. But under the cover of darkness he switched it for water and diluted bleach which would cause no real harm.
It was during this time his son Mohammed, an officer at a military academy, was arrested. The mukhabarat, or secret police, made him sign papers admitting to crimes he had not committed, Brig Gen Sakat said.
“I thought that it was a way to pressure me maybe, or to scare me in order not to become a defector. Maybe that was the plan of the regime,” he told The Telegraph.
Brig Gen Sakat paid for his son’s release and a month later Mohammed was returned, weighing just 7½ st and bearing the signs of torture.
But the attacks were claiming few victims and after the third time the regime became suspicious. It was then he decided to escape to Jordan and then on to Turkey, where he joined the opposition Free Syrian Army.
“I couldn’t believe at the beginning that Assad would use these weapons on his people,” he said.
Syria’s chemical weapons programme was started in the early 1980s with the aim of defending against enemy states such as its neighbour, Israel. “I could not stand and watch the genocide. I couldn’t hurt my own people,” he said.
Brig Gen Sakat has had several attempts on his life since defecting and has given few interviews since leaving Syria.
He now works documenting chemical attacks from outside the country, sharing evidence and information from local activists with the OPCW.
He said he does not think Assad will give up the remaining stockpile as long as he is in power.
“He will not let go of the chemical weapons while he is leader of Syria,” the general said. “If he is forced to leave, he might confess to where some of it is hidden only so it doesn’t end up in the wrong hands.”