Arriving for talks with G7 foreign ministers in Lucca in Italy, Mr Johnson issued a fresh appeal to Russian president Vladimir Putin to abandon his backing for his Syrian ally following last week's chemical weapons attack on a rebel-held town.
He said 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," Mr Johnson told reporters.
Russia and Syria could face 'further sanctions', warns Boris Johnson
Countries that support and oppose the missile strikes in Syria
Mon, April 10, 2017
European countries have been broadly supportive so far as President Trump ordered missile strikes against a Syrian airfield from which a deadly chemical weapons attack was launched
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SYRIA OPPOSES: President Bashar al-Assad's office denounced the U.S. strike as 'reckless, irresponsible behavior.'
The Foreign Secretary said US President Donald Trump's decision 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 had to decide which side they were on.
We will be discussing the possibility of further sanctions certainly on some of the Syrian military figures
Boris Johnson – Foreign Secretary
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"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 his Russian counterpart Sergey Lavrov, saying that it was important US Secretary of State Rex Tillerson had the "clearest possible mandate" when he went to Moscow to deliver the response of the G7.
"It think 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," he said.
G7 foreign ministers in Lucca, Italy
Canadian Prime Minister Justin Trudeau
Earlier, Downing Street re-iterated the Government's support for the US missile strike while calling for a renewed diplomatic push to end Syria's six-year civil war.
However, Prime Minister Theresa May's official spokesman refused to be drawn on whether the UK would support further US military action, saying it was a "hypothetical question".
Russia and Iran – Assad's two principal international backers – warned on Sunday that they would respond "with force" to any fresh attack on their ally.
Amid the heightened international tensions, Downing Street called on all sides to work to find a peaceful solution to the conflict.
"What we are focused on is building international support for a political solution to end the conflict and bring lasting peace and stability to Syria," Mrs May's spokesman said.
"We are calling on Russia and Iran to do everything possible to bring about this settlement and to work with the international community to ensure last week's shocking events aren't repeated."
Trump ordered a series of missile strikes last week in response to the deaths of more than 80 people
The Prime Minister, who is on a short walking holiday in Wales, discussed the latest developments in a telephone call with Canadian Prime Minister Justin Trudeau on Sunday evening.
Pressed on whether the UK would back further US action if the regime were to repeat its use of chemical weapons, her spokesman said: "What they have said is that they are not planning any further strikes but they could re-consider if they (the Syrians) use chemical weapons again. Until that point it is a hypothetical question."
President Trump ordered a series of missile strikes last week in response to the deaths of more than 80 people, including children, during a chemical attack in the rebel-held town of Khan Sheikhoun.