Leading up to the USA missile attack, Trump's administration had said that Assad's future was up to the Syrian people.
Russian Federation has rejected accusations that Assad used chemical arms against his own people and has said it will not cut its ties with Assad, who has been locked in a six-year-old civil war that has devastated Syria and displaced half its population.
"What we're trying to do is to give Rex Tillerson the clearest possible mandate from us as the West, the United Kingdom, all our allies here, to say to the Russians "this is your choice: stick with that guy, stick with that tyrant, or work with us to find a better solution", Johnson said after meeting Tillerson. Since then, members of the Quds force have taken on training and advisory roles as well as fighting alongside their Shiite allies Hizbollah.
He said another chemical weapons attack could not be allowed to happen, adding: "It is clear to us the reign of the Assad family is coming to an end".
Even since the missile strikes, signals have been mixed.
"We must have a dialogue with Russian Federation", he said.
The two powers traded further jabs on Tuesday.
Johnson on Monday called on Moscow to do "everything possible to bring about a political settlement in Syria and work with the rest of the global community to ensure that the shocking events of the last week are never repeated".
U.S. defence secretary Jim Mattis said on Monday that the 59 Tomahawk missile strikes had damaged or destroyed 20 per cent of Syria's operational aircraft, as well as fuel and ammunition sites.
We want to create a future for Syria that is stable and secure.
Sky's Foreign Affairs Editor Sam Kiley, in Moscow, said: "From the Russian perspective, what this window of opportunity that Washington and London are talking about, is extremely vague".
They bring Russia's official death toll in the country to 29. France said it was not discussed in depth.Western countries have been calling for Assad to leave power since 2011, the start of a civil war that has killed at least 400,000 people and created the world's worst refugee crisis.Assad's position on the battlefield became far stronger after Russian Federation joined the war in support of him in 2015.
"The strike launched by the United States on Shayrat Airfield in Syria had the understandable intention to prevent and deter the spread and use of chemical weapons and was limited and focused on this objective", they said in a joint statement after a summit in Madrid. But this distinction doesn't much matter to the dead.
"But only Russian Federation can answer that question". His selection for secretary of state suggested a new detente. Moscow is a staunch ally of President Bashar Assad whom the United States blamed for the chemical attack. The official said the presence of the surveillance drone over the hospital couldn't have been a coincidence.
Trump had previously appeared disinclined to intervene against the Syrian leader and the attack raised expectations that he might now be ready to adopt a tougher-than-expected stance with Russia, Assad's main backer.
That still leaves questions about how hard the United States is working to remove Mr Al Assad from power.
"Now is the time for them to recognise that and help the rest of the world, and above all help the people of Syria, to move forward". Diplomats gathered in Italy as US officials in Washington floated the possibility of new sanctions on the Syrian and Russian military, plus the threat of additional USA military action if Assad's government continues attacking civilians.
"I think it is clear that we see no further role for the Assad regime longer-term given that they have effectively given up their legitimacy with these type of attacks", he said before flying to Moscow.