The British foreign secretary’s warning to President Bashar al-Assad came as his forces continued to launch attacks on the besieged, rebel-held region of Eastern Ghouta.
Paris and Washington also both said they would back targeted military action against Damascus if there were proof chemical weapons had been used by forces loyal to Assad.
The international chemical weapons watchdog was on Tuesday reported to have launched an investigation into attacks in Syria’s Eastern Ghouta, hours after Boris Johnson said the UK would consider bombing the Syrian government if it was proved to have used gas on civilians.
Johnson said he hoped Britain would not stand by in the event of a chemical attack, voicing support for limited strikes if there is “incontrovertible evidence” of Syrian government involvement.
“If we know that it has happened, and we can demonstrate it, and if there is a proposal for action where the UK could be useful then I think we should seriously consider it,” Johnson told BBC Radio 4’s Today programme.
Welcoming @BorisJohnson intervention on Syria this morning, that we could support US unilateral strikes in reaction to chemical weapons use. Remains a stain that the level of human suffering can continue unchecked – we must take a stand for what we believe in. pic.twitter.com/HoFaKOnqra
— Johnny Mercer MP (@JohnnyMercerUK) February 27, 2018
Several hours later the Organisation for the Prohibition of Chemical Weapons (OPCW) was reported to have launched an investigation into attacks on the region to determine whether banned munitions had been used, diplomatic sources told Reuters.
The news agency reported that the OPCW would examine attacks including one on Sunday which health authorities said killed a child and caused symptoms consistent with exposure to chlorine gas, the sources said.
The OPCW mission is seeking to determine whether chemical weapons were used in violation of the international weapons convention, which Syria signed in 2013 after hundreds died in a huge sarin gas attack in the same area.
It comes as Syrian jets continued to strike Eastern Ghouta on Tuesday, despite a Russian call for a five-hour daily truce to allow the 400,000 people living there under siege to leave.
Emily Thornberry, the shadow foreign secretary, said Assad’s bombing and the use of chemical weapons were a “war crime”, adding that there must be a “reckoning for those responsible”.
But she stopped short of backing military action and said Labour opposed “yet more military intervention, I believe that would simply prolong and deepen the war”.