Assad gave the interview to AFP in Damascus on Wednesday.
Eighty-nine people were killed and others injured in the April 4 chemical attack in Idlib province.
The new measure would require "full cooperation with the investigation" into the apparent attack that took place in the rebel-held town of Khan Sheikhun, British Ambassador Matthew Rycroft said on Twitter.
Images of children foaming at the mouth shocked the world last week. Sufferers were taken to hospitals across the border in Turkey.
Turkish health officials on Tuesday said postmortem tests on victims of the suspected attack had also concluded that the deadly sarin nerve agent had been used. A showdown vote by the Security Council is planned for later Wednesday on a proposed United Nations resolution that would compel the Syrian regime to provide helicopter pilots and flight logs from the day of the attack to outside investigators.
"Definitely, 100 percent for us, it is fabrication".
"This story is not convincing by any means", he said.
The OPCW has blamed Assad's government for at least two attacks in 2014 and 2015 involving the use of chlorine. "That is our position on the use of chemical weapons", he said.
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The Syrian opposition has reported that Assad's forces have begun flying again from an air base struck last week by US missiles. Prime Minister Malcolm Turnbull said the Assad regime must be brought into line after last week's chemical gas attack.
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He told Agence France-Presse this week that he wasn't even confident the dead children shown on video were "dead at all". Their timing and location have been verified by independent researchers.
Mr Assad insisted that it is not clear yet what happened during the incident, since evidence touted by the West comes from "a branch of al-Qaeda".
UK Prime Minister Theresa May said on Thursday it was "highly likely" the Assad regime was behind the attack. He claimed details of the attack were made up in order to give the U.S. an excuse to bomb a Syrian airbase in retaliation.
Turkey, which treated numerous wounded, says it has "concrete evidence" Sarin was used.
Mr Assad's claim comes a day after Russian Federation vetoed a UN Security Council resolution demanding a swift investigation into the attack.
Assad said Syria would only allow an "impartial" investigation into the poison gas incident.
Syriac Catholic Patriarch Ignace Joseph Younan described the USA missile strike as an "aggression" and castigated the US administration for not waiting for the U.N.to conduct an investigation on what was believed to be a chemical air strike.
In northern Syria, U.S. -backed Kurdish forces announced they launched the fourth stage of their operations against the Islamic State group, amid reports from activists that the group has suffered a spate of casualties.