Johnson reiterated that in attacking Syria "the United States has acted with the full support of the British Government", adding that the "only one conclusion" that could be reached based on the known evidence of the alleged chemical attack in rebel-held Idlib province is that "the Assad regime nearly certainly gassed its own people".
French Foreign Minister Jean-Marc Ayrault said French intelligence services would provide proof of that in the coming days.
Israeli defense officials say Syrian President Bashar Assad still has up to three tons of chemical weapons.
Mr Assad claims the poison gas attack is a fabrication used to justify the United States air strike.
Assad stressed that the video clearly shows people without any means of chemical warfare protection helping those who allegedly fell victims to sarin gas attack.
Earlier this week, Organization for the Prohibition of Chemical Weapons (OPCW) said that it is identifying sarin in samples related to the April 5 incident.
United States forces had given Moscow a heads up shortly before firing off 59 Tomahawk cruise missiles at Shayrat, where Russian personnel and equipment are based under Moscow's ongoing military intervention to prop up Assad. Even if Syrian planes are not parked in Russian-controlled areas, any strike on them could endanger Russian lives and equipment due to their proximity.
Konashenkov questioned how the OPCW, which he said was created as an objective and impartial worldwide organization, was able to come to that conclusion in two weeks while still being unable to identify mustard gas use in Aleppo in four months.
The Syrian government has been locked in a six-year civil war against an array of opposition forces.
Syrian President Bashar al-Assad still possesses up to three tons of chemical weapons, Israeli defense officials have said.
A senior military official told reporters that the Israeli intelligence estimates that Assad has "between 1 and 3 tons" of chemical weapons.
By tightly restricting the delivery of food and medical relief, the government has secured the surrender of broad swathes of the opposition-held countryside around Damascus, as well as the former opposition enclaves in Aleppo and Homs, Syria's first and third largest cities, respectively.
Speaking in Washington during a news conference with Italian Prime Minister Paolo Gentiloni, U.S. President Donald Trump said the shooting "looks like another terrorist attack" and sent condolences to France.
"We don't know whether those dead children were killed in Khan Sheikhoun", he said in an interview with AFP TV.
As for the impact of that incident, the adviser said that the recent U.S. attack on Sheyrat's Syrian airbase in the province of Homs had a political rather than a military objective, insisting that Syria will respond to this "aggression" in the appropriate time.
In this handout provided by the US Navy, the guided-missile destroyer USS Ross fires a Tomahawk land attack missile April 7, 2017 in the Mediterranean Sea.