Boris Johnson has offered Russia a chance to end theSyria crisis, saying “they still have time to be on the right side of the argument” as he warned that the United States may strike again.
The Foreign Secretary also branded Syrian president Bashar al-Assad an “arch-terrorist” and pledged that the UK and allied forces would gather evidence for “war crimes prosecutions for those responsible”.
Mr Johnson went on the front foot after he was criticised for failing publicly to persuade G7 countries to agree sanctions against Russia for its support of the Assad regime.
Relations with Moscow were strained when President Donald Trump ordered a missile strike against the Syrian airfield from which a chemical weapons attack was launched on civilians.
The gas attack had killed 70 people, many of them children who were filmed dead or dying, frothing at the mouth.
Mr Johnson then cancelled a visit to Moscow and threatening sanctions against Russia and Syria, a promise which backfired when the Germans and Italians refused to agree until an investigation has been carried out into who was to blame for the nerve gas attack in Idlib province.
In an article for The Telegraph, his first wide-ranging remarks since the row, Mr Johnson appealed to Moscow to help stop the killing.
He said: “Assad uses chemical weaponsbecause they are not only horrible and indiscriminate. They are also terrifying.
"In that sense he is himself an arch-terrorist, who has caused such an unquenchable thirst for revenge that he can never hope to govern his population again.
“He is literally and metaphorically toxic, and it is time Russia awoke to that fact. They still have time to be on the right side of the argument.”
Mr Johnson said there was no doubt that a bomb dropped from two Syrian U-22 planes which “took off from an airbase where chemical weapons have been stored” were behind the attack.
He said: “British scientists have analysed samples from the victims of the attack. These have tested positive for sarin or a sarin-like substance.
“The UK, the US and all our key allies are of one mind: we believe that this was highly likely to be an attack by Assad, on his own people, using poison gas weapons that were banned almost 100 years ago, under the 1925 Geneva protocol.”
Russia hosted foreign ministers from Iran and Syria late last week in a show of support for Assad’s government days after the US missile strike.
The Russians could “join a coalition of more than 60 countries in the fight against Daesh, to maintain their strategic interests in Syria, with the prospect of more productive relations with President Trump and in the knowledge that the West will eventually help rebuild the country”, Mr Johnson said.
He added: “In exchange they should commit to produce a real ceasefire, to end the use of chemical weapons and barrel bombs, and to bring about a political settlement that relieves the Syrians of the tyranny of Assad.
“The Russians saved him. The Russians can help remove him, through a carefully supervised transition process that preserves key institutions of state – and usher in a stable and pluralist future for the country.”
Mr Johnson received a boost late last week when his French counterpart Jean-Marc Ayrault jointly urged the international community to make sure those responsible for the Syrian chemical weapon attack are held to account.
Mr Johnson's alliance with Mr Ayrault was seen as a sign that he is working to build support, having appeared isolated over the issue of sanctions after the G7 meeting.
Mr Johnson also put Assad on notice that the US stood ready to launch more attacks.
He said: “America has struck and could of course strike again. That alone creates an ambiguity that should prey on the guilty minds of Damascus.”
Mr Johnson said it was vital that the Russian people understood “the horrific nature of the regime they are backing in terms they cannot fail to understand".
He added that the current crisis "is, in fact, an opportunity for Russia. Moscow has reached the high point of its influence in Syria.
“They still have innumerable rebel groups to subdue, and they find themselves in a league of super-villains with Hezbollah and Assad. Is that what they want?”
He said that “now is surely the moment for them to make a sensible compromise”.
Mr Johnson does not rule out further British military intervention in Syria, but stresses this is unlikely given the fall-out from the British and American invasion of Iraq in 2003.
He said: “We all know that we are a very long day’s march from any large-scale deployment, any major western engagement in Syria.
“The lessons of the 2003 invasion of Iraq are painful, and they understandably affect politicians and public on both sides of the Atlantic.
“Which means that we should instead focus relentlessly on the reality of what Assad has done: killed innocents with a banned and abominable weapon.”
The news came as at least 24 people were killed in an explosion that hit near evacuation buses in northern Syria in an area controlled by opposition fighters.
The car bomb on Saturday hit at an evacuation point south of Aleppo city where dozens of buses have been parked for over 30 hours as a much-criticised population transfer deal stalled.
The Britain-based Syrian Observatory for Human Rights said at least 24 people were killed, including rebels who guarded the bus and passengers.