The Foreign Secretary insisted it was "highly likely" Assad was responsible for the chemical weapons atrocity as Johnson insisted America "could of course strike again".
Last week's bombing in the town of Khan Sheikhoun in the rebel-held province of Idlib near the Turkish border was the most lethal since a sarin attack on August 21, 2013 killed hundreds in a suburb of the capital, Damascus.
In his AFP interview, Assad accused the West, particularly the United States, of being "hand-in-glove with the terrorists" over the Khan Sheikhoun incident.
Syrian President Bashar al-Assad said his government handed over all its chemical weapons stockpiles in 2013 and could not have been behind last week's suspected sarin attack.
The problems of the Syrian Civil War will not be solved overnight, but preventing the use of chemical weapons in it is good for civilians there and for the long term goal of keeping the worldwide consensus on the ban of chemical weapons strong. Syria "provided a grossly incomplete declaration to the OPCW of its chemical weapons program".
Assad also claimed that the alleged chemical attacks in Khan Sheikhoun were all a "fabrication" to give the USA military justification for a strike.
Lebanon's state-run National News Agency says security force have arrested one of the most wanted men involved in "terrorism" and in backing al-Qaida's branch in Syria. "They fabricated the whole story in order to have a pretext for the attack", he said.
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.
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Syria joined the Organization for the Prohibition of Chemical Weapons in 2013 under severe global pressure after a deadly chemical attack on a Damascus suburb.
Experts tell NPR this explanation does not make sense, based on the nature of the chemicals used. "It continues to possess and use chemical weapons".
Russian Federation and Iran hailed the readiness of the Syrian government to accept such a mission, Lavrov said.
Earlier this week, Turkish doctors also said that test results conducted on victims confirmed that sarin gas was used.
Uzumcu said on Thursday he expected its fact-finding mission into the reported Khan Shaykhun attack to complete its work within the next two to three weeks. The organization has not revealed any details, citing the need to preserve the integrity of the investigation and the safety of the organization's staff.
The remarks came on the same day that Syrian Foreign Minister Walid al-Muallem, along with his Iranian and Russian counterparts Mohammad Javad Zarif and Sergei Lavrov, met in Moscow to discuss the recent U.S. missile strikes on an air base in Syria's central province of Homs.
He said Russian Federation vetoed a Western draft United Nations resolution Wednesday because it failed to mention the need to inspect the area of the attack.
Furthermore, the US response to the strike was an attempt to send a message to the world that the use of chemical weapons will not be allowed to become an international norm.