Speaking in the Commons, the Foreign Secretary argued there was “no military solution that we can impose” to bring peace to war-torn Syria.
But Mr Johnson agreed there was a case for “limited strikes” if there was “incontrovertible evidence of further use of chemical weapons by the Assad regime or their supporters”.
He said many people believed the US “did the right thing” when it launched a cruise missile attack on a Syrian airfield last April in response to chemical weapon use. “I will certainly hope that the West does not stand idly by”, Mr Johnson told MPs.
The urgent question came after a week in which Syria’s army and its allies have inflicted one of the heaviest bombardments of the seven-year war on eastern Ghouta, an enclave outside Damascus, killing hundreds.
Local doctors and monitors have said a suspected chlorine attack in the rebel-held area left 18 people injured on Sunday evening.
The week-long carnage in the area has killed more than 500 people in air strikes and shelling by forces loyal to Assad.
A United Nations security council resolution was approved unanimously on Saturday, calling for a month-long ceasefire “without delay”, but the killing continued.
On those attacks, Mr Johnson said: “The House will have noted the disturbing reports of use of chlorine gas. I call for these reports to be fully investigated and for anyone held responsible for using chemical weapons in Syria to be held accountable.”
But, when it was pointed out that both Angela Merkel and Emmanuel Macron have contacted Vladimir Putin, Mr Johnson was unable to say if Theresa May had also had a “recent” conversation.
Emily Thornberry, the Shadow Foreign Secretary, called the indiscriminate bombing in Syria and the use of chemical weapons “a war crime”, adding that there “must be a reckoning for those responsible”.
But she said Labour opposed “yet more western military intervention, I believe that would simply prolong and deepen the war”.
“Ultimately, we can only end this dreadful conflict and the suffering of all the Syrian people through genuine peace talks involving all non-jihadi parties and the agreement of a political solution,” Ms Thornberry told MPs.
The question was sponsored by Labour MP John Woodcock, who condemned Britain and other Western countries for failing to act on the appalling bloodshed in Syria.
“The men and women of Ghouta who lie in pieces, deliberately targeted by Assad’s Russia-enabled bombs, the dead children whose faces are altered by the chlorine gas that chocked them.
“They should not be strewn in the rubble of Eastern Ghouta. Those bodies should be piled up in this chamber and lain at the feet of governments of every single nation which continues to shrug in the face of this horror.”
Mr Woodcock said the UN Security Council “is broken while one of its permanent members flouts the basic laws and systems of order it was created to uphold”. “In these dreadful circumstances, being cowed into inaction by this strangulated body is a greater violation than seeking to act even without its own authorisation,” he added.
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