Boris Johnson has called on Russia to end its "seemingly indefensible" support for Syrian president Bashar Assad's "killing machine" and help pave the way for an opposition plan to end the country's brutal conflict.
Ahead of a Wednesday meeting with the Syrian High Negotiations Committee (HNC) opposition group, the Foreign Secretary called on Russia and the United States to help create a ceasefire.
That would allow the HNC, the "broadest-based" opposition group in Syria, to bring forward its plan for a post-Assad Syria that is "democratic and pluralistic" but which does not want to "sweep away all the existing structures of the state".
Mr Johnson's comments ahead of the Westminster meeting could be seen as a signal that the Government is ready to offer stronger backing to the Syrian opposition.
The talks will take place against a backdrop of alleged chlorine bomb attacks in Aleppo.
Mr Johnson wrote in The Times: "Even the Russians have accepted that there must be political transition. But then the Russians are also employing their military muscle to prevent him (Assad) from losing and to keep him in power.
"When the Russians are asked to explain this seemingly indefensible conduct they reply with one stubborn question: the question with which we began. What then? What follows Assad?"
Mr Johnson highlighted the HNC plan for a six-month negotiating phase between the regime and the opposition with a total ceasefire and humanitarian access.
This would be followed by an 18-month period of transitional rule with a mix of opposition figures and current government and civil society representatives.
Assad would be removed from power and there would then be elections, Mr Johnson said.
"There will be people meeting in London who have direct experience of running Syria, but who utterly reject the Assad-style police state," the Foreign Secretary wrote.
"They want to create a new country in which there are checks and balances in government and in which the rights of women and minorities are respected.
"Their ambition is to ensure a safe space, free from terror, to which migrants can return.
"Above all, the High Negotiations Committee does not represent the victory of one sectarian group over another or the transfer of power from one faction in Syria to another.
"They propose a gradual transition."
He added: "There is still a chance that this vision can be made to work. If the Russians and Americans can together create a ceasefire, then the talks can restart in Geneva with the difference, perhaps, that all sides will by then have seen at least the scaffolding of a post-Assad Syria."