Meanwhile, the alternative press is reporting that Assad actually attacked an al-Qaida storage facility in an attempt to destroy their ammunition, not knowing that non-sarin chemicals were stored there.
Two weeks passed after the alleged use of chemical weapons in Khan Sheikhoun.
"We have elements that will enable us to show that the regime knowingly used chemical weapons", he said.
Johnson also commanded UK's own efforts in investigating the chemical incident in Khan Sheikhoun, claiming that it is "thanks in large measure to UK diplomacy, the United Nations now has a joint investigative mechanism with a mandate to identify any party responsible for chemical attacks in Syria".
Ahead of that disarmament, Assad's government disclosed it had some 1,300 tons of chemical weapons, including sarin, VX nerve agent and mustard gas. But it has carried out a number air strikes against suspected arms shipments bound for Assad's ally Hezbollah in Lebanon, and in retaliation to errant fire into the Golan Heights.
Rex Tillerson Accuses Tehran Of 'Alarming Provocations'
As part of a long list of charges, he criticised Iran's involvement in the Syrian conflict and its support for President Bashar al-Assad.
It will be "very difficult" for the United Kingdom government to refuse military assistance to the USA in Syria if they expressly ask for it, British FM Boris Johnson has said, responding to an MP's question on if he intends to heed parliament's vote against the incursion.
The entire stockpile was said to have been dismantled and shipped out under worldwide supervision in 2014 and destroyed.
"It is my belief, though I stress no such decision has yet been taken, that were such a request to be made in future, were it to be a reasonable request in pursuit of similar objectives, then I think it would be very hard for the United Kingdom to say no", the Independent quoted Johnson as saying.
A fact-finding mission from the Organization for the Prohibition of Chemical Weapons, an global watchdog, is investigating the April 4 incident and on Wednesday its director-general, Ahmet Uzumcu, said that the results "from four OPCW designated laboratories indicate exposure to sarin or a sarin-like substance".
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