The regime of president Bashar Assad has "every reason to be worried" about the development of the uprising in Syria, Prime Minister David Cameron has said.
Mr Cameron urged Russian president Vladimir Putin to put his weight behind international efforts to step up the pressure on Damascus when the two men met at the G20 summit in Mexico today.
But the Prime Minister cautioned against Mr Putin's proposal to invite Iran to an international conference Russia is hoping to organise to discuss the unrest in Syria.
Russia and China have so far blocked international proposals for more vigorous action to pave the way to a political transition which would see Assad removed from power.
Speaking ahead of his meeting with Mr Putin, Mr Cameron said the position in Syria was changing as predictions of a swift and easy victory for Assad's troops over rebel forces proved unfounded.
"What is happening on the ground is that while some people predicted the Syrian army would crush the rebels, that has been shown so far not to be the case," said Mr Cameron.
"And I think the regime in Syria has every reason to be worried, but it really does require Russian and Chinese buy-in to a plan."
Mr Cameron said Britain supported Russian proposals for an international conference, but not the idea of inviting Syria's neighbour Iran - a key backer of the Damascus regime.
He reiterated British support for the peace plan drawn up by UN envoy Kofi Annan, which envisages a ceasefire leading to a transition of power.
"I don't think Iran attending a conference is the right approach," said Mr Cameron. "We're not opposed to a conference so long as it has agendas and outcomes. The key is to work on that bit of the Annan plan which could help to deliver political change at the top of Syria."