Denying any involvement whatsoever, the Syrian leader said: "A hundred percent for us, it's fabrication".
She said a team of British scientists had examined material from the Khan Sheikhoun area and concluded it was "very clear" that sarin gas or a similar substance had been used.
The interview with AFP's Damascus bureau chief took place on Wednesday.
Syria denied any use of chemical weapons and Russian Federation backed its claim.
The Syrian president also challenged accounts of alleged chemical attack provided by "White Helmet" volunteer medics.
He said if Syria had chemical weapons, it would have used them against the "terrorists" who were advancing near the capital, Damascus.
The agency said Thursday that Jalal Mansour was detained in northern Lebanon after changing his looks and using a fake identity card for months.
Washington blamed the Syrian government for the attack. "It could happen anytime, anywhere, not only in Syria".
"We don't know whether those dead children were killed in Khan Sheikhoun", the tyrant said.
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In an exclusive video interview with Agence France-Presse, he said "there was no order to make any attack".
'The UK, the USA and all our key allies are of one mind: we believe that this was highly likely to be an attack by Assad, on his own people, using poison gas weapons that were banned nearly 100 years ago, under the 1925 Geneva protocol'.
In Moscow, Russian Foreign Minister Sergey Lavrov said Thursday that inspectors from the chemical-weapons watchdog should visit both Khan Sheikhoun and the Syrian air base, which the USA said served as a platform for the attack, to get a full and objective picture.
"Our firepower, our ability to attack the terrorists hasn't been affected by this strike".
CNN and other global media interviewed victims of the attack who described how many of their relatives were killed. He was pictured cradling their dead bodies.
"The only information the world have had till this moment is published by al Qaeda branch", he said.
Assad said he would only allow an "impartial" external investigation involving "unbiased countries", according to AFP.
Turkish President Recep Tayyip Erdogan says he and Russian President Vladimir Putin have agreed to support an investigation by the Organization for the Prohibition of Chemical Weapons into the use of chemical weapons in northern Syria.
It was the 8th time Russian Federation has used its veto power.