BORIS Johnson has warned senior Russian military officers involved in co-ordinating the “toxic” Assad regime's repression of the Syrian people that they could face international sanctions.

The Foreign Secretary, meeting his G7 counterparts in Italy, urged Vladimir Putin, the Russian President, to abandon his support for his Syrian ally following last week's chemical weapons attack on a rebel-held town, which has been condemned by western leaders as barbaric.

He said that if the Russians continued to support the regime in Damascus, they would be "contaminated" by its actions and could find themselves the target of new international sanctions.

"We will be discussing the possibility of further sanctions certainly on some of the Syrian military figures and indeed on some of the Russian military figures who have been involved in co-ordinating the Syrian military efforts and are thereby contaminated by the appalling behaviour of the Assad regime," explained Mr Johnson.

The Secretary of State claimed that the decision by Donald Trump to launch cruise missiles against a Syrian air base in response to the regime's use of sarin nerve agent had "changed the game" and the Russians now had to decide which side they were on.

"They have a choice. That choice is to stick like glue to the Assad regime - that toxic regime which poisons its own people and is indeed poisoning the reputation of Russia - or to work with the rest of world to find a political solution," he said.

Mr Johnson defended his decision to pull out of talks with Sergey Lavrov, his Russian counterpart, saying it was important that Rex Tillerson, the US Secretary of State, had the "clearest possible mandate" when he went to Moscow on Wednesday to deliver the response of the G7.

"It is very important in these circumstances for the world to present a united front and for there to be absolutely no ambiguity about the message," declared Mr Johnson.

But Jeremy Corbyn cautioned that a "proxy war" was now being threatened between the US and Russia over Syria.

The Labour leader also accused Mr Johnson of "encouraging further bombing" by suggesting the US President could order more missile strikes against the Syrian regime.

"Surely we have to get Russia and America around the table together to put pressure on respective parties to this war because it's in danger of becoming a proxy war between them, in order to have a meaningful and effective ceasefire."

He added: "If one side strikes and another side retaliates therein lies the road to something utterly disastrous."

Meantime, Tim Farron, the Liberal Democrat leader, urged Theresa May, currently on a walking holiday in Wales, to get involved.

"Everyone deserves a break from time to time and Snowdonia is a beautiful part of the country but, ultimately, there are some things too important to be left to the court jester. The situation in Syria is one of them and I would have hoped that the Prime Minister would have shown some leadership on this issue and co-ordinated Britain's response herself," said the Cumbrian MP.

Earlier, Downing Street re-iterated the UK Government's support for the US missile strike while calling for a renewed diplomatic push to end Syria's six-year civil war.

As tensions continue to mount between the West and Russia and Iran over last week’s chemical attack and the US Government’s military response to it, the Russian Embassy in London tweeted a poll, noting: “If G7 ultimatum to Russia brings us to real war, what is your trust in @realDonaldTrump as a wartime leader&@BorisJohnson as his lieutenant?”

Mrs May’s spokesman, asked what her response to this was, said: “What we are focused on…is building international support for a political solution to end the conflict and bringing lasting peace and stability to Syria.

“Our focus is calling on Russia and Iran to do everything possible to bring about this political settlement and to work with the international community to ensure last week’s shocking events aren’t repeated.”

Asked if the bombing ordered by Mr Trump was in accordance with international law, the spokesman replied: “We have said we support it and it was appropriate.”