Assad attempts to lie about children dying
Shocking video evidence showed dead bodies and injured victims, mostly children, shaking while foaming at the mouth, sparking global outrage.
In an exclusive video interview with AFP, Assad insisted his regime handed over all chemical weapons stockpiles in 2013, under a deal brokered by the regime's key ally, Russia, to avoid threatened US military action.
"There was no order to make any attack, we don't have any chemical weapons, we gave up our arsenal a few years ago," Assad claimed.
But the evidence is stacked up against him.
The attack on the rebel-held town on April 4 killed at least 89 people, including 31 children, and saw hundreds suffer symptoms consistent with a nerve agent.
Assad claimed those images - which emerged from numerous sources inside Syria - were all "fake".
"You have a lot of fake videos now," Assad said.
"We don't know whether those dead children were killed in Khan Sheikhun. Were they dead at all?"
British scientists and Turkish health officials who carried out post-mortem tests on victims, confirmed this week that a sarin-like substance was used in the attack on Khan Sheikhun.
Yet Assad insisted: "Definitely, 100 percent for us, it's fabrication."
"Our impression is that the West, mainly the United States, is hand-in-glove with the terrorists. They fabricated the whole story in order to have a pretext for the attack."
Assad's key ally, Moscow, said the deaths had been the result of a conventional strike hitting a rebel arms depot containing "toxic substances".
Again, weapons experts have said that, if that were the case, much of the sarin would have been instantly destroyed, with the rest dispersed over a much smaller area.
Weaponised sarin releases the agent as an aerosol, meaning tiny droplets travel much further, affecting many more people, as was the case in Khan Sheikhun, where whole families were reportedly found dead in their beds.
Last week's attack was not the first time the Syrian regime has used toxic weapons.
In November, a joint investigation by the UN and the Organization for the Prohibition of Chemical Weapons (OPCW) found that several units of the Syrian regime's forces had used chemicals against three villages in northern Syria in 2014 and 2015.
It was the first time an international probe blamed Bashar al-Assad's forces after years of denial from Damascus.
On Wednesday, Moscow vetoed a draft UN resolution demanding the Syrian government cooperate with an investigation into the attack - describing the measure as "unacceptable".
It was the eighth time that Russia has used its veto power to block action directed at Damascus.
British Foreign Secretary Boris Johnson said the veto "puts Russia on the wrong side of the argument", while French President Francois Hollande warned Russia it "bears a heavy responsibility" for continuing to protect Assad.
In a show of continued support for the regime, Moscow will host Syrian Foreign Minister Walid Muallem for talks with Lavrov on Thursday.
On Friday the two will join a three-way meeting with Foreign Minister Mohammad Javad Zarif of Iran, another key ally of the Assad regime.
The Syrian conflict began when the Baath regime - in power since 1963 and led by President Bashar al-Assad since the death of his father - responded with military force to peaceful protests demanding democratic reforms during the Arab Spring wave of uprisings, triggering an armed rebellion fuelled by mass defections from the Syrian army.
According to independent monitors, hundreds of thousands of civilians have been killed in the war, mostly by the regime and its powerful allies, and millions have been displaced both inside and outside of Syria.
The brutal tactics pursued mainly by the regime, which have included the use of chemical weapons, sieges, mass executions and torture against civilians have led to war crimes investigations.