Russian President Vladimir Putin is the "one man on this planet" who can end the Syrian civil war, according to the Foreign Secretary.
Speaking on The Andrew Marr Show, Philip Hammond said: "Whether or not Assad goes or stays ultimately will depend on whether the Russians are prepared to use their influence to remove him.
He added: "There's one man on this planet who can end the civil war in Syria by making a phone call, and that's Mr Putin."
He went on to say there were around 150,000 moderate opposition forces in Syria, which are now suffering attrition from Russian air strikes.
"The Russians have launched ferocious air attacks, rapidly increasing the intensity of them over the last few weeks, and that has forced them [Syrian opposition forces] out of some of the positions they control.
"But the important thing is the Syrian regime does not have the forces, does not have the strength and the organisation, to take control of those areas. So it's a bit of a stalemate."
The Foreign Secretary also called on the Russians to stop air strikes in Syria, which he said consisted of "carpet bombing tactics" and "indiscriminate bombing of civilian areas".
"We demand that the Russians comply with their obligations under international law and their obligations under UN Security Council resolutions that they have signed up to."
Talks in the German city of Munich on Friday saw world powers agree to a temporary "cessation of hostilites" within a week, but there seemed to be little hope of a long-term truce after Syrian President Bashar al-Assad vowed to regain control of the entire country by force.
In a statement released in response to the ceasefire plan, Mr Hammond said: "This will be an important step towards relieving the killing and suffering in Syria. But it will only succeed if there is a major change of behaviour by the Syrian regime and its supporters.
"Russia, in particular, claims to be attacking terrorist groups and yet consistently bombs non-extremist groups including civilians.
"If this agreement is to work, this bombing will have to stop: no cessation of hostilities will last if moderate opposition groups continue to be targeted."
Over 250,000 people have died in the Syrian civil war, which has caused a refugee crisis and empowered Isis militants.
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