A Russian official has said it is "impossible" to organise an international conference by the end of this month which eventually aims to bring together the Syrian government and opposition for negotiations.
Washington and Moscow announced joint plans earlier this week for such a summit, which would work towards ending the country's bloody civil war.
The Russian official, who was at yesterday's talks between President Vladimir Putin and Prime Minister David Cameron, said: "By the end of May is impossible".
The anonymous official said disagreements remained about who could take place in the peace process.
Intense fighting is forcing increasing numbers of families to flee suburbs of the Syrian capital Damascus.
The Yarmouk district of the city was once home to three quarters of a million people. Now, perhaps 10,000 remain.
ITV News International Editor Bill Neely reports:
The Prime Minister was given a brief tour of the stadium in Sochi where the Winter Olympics are to be held in 2014 by Russia's President Vladimir Putin.
David Cameron has made a statement after concluding talks with Vladimir Putin in Sochi, Russia.
The prime minister said: "We have had very substantive, very purposeful, very useful talks today.
"We discussed the appalling and deteriorating situation in Syria. Millions of lives lost, millions more fleeing their homes.
"The history of Syria is being written in the blood of their people. It's no secret that we've had differing views on how to handle the situation.
"But we share fundamental aims to end the conflict, to stop Syria fragmenting, to let the Syrian people choose who governs them, and to prevent the growth of violent extremism."
David Cameron is currently meeting with Russian president Vladimir Putin, discussing the need for diplomatic progress to end the conflict in Syria.
David Cameron meets the Russian President in the sub tropical Black Sea resort of Sochi today.
Amidst the palm trees you’d never believe this place is hosting the Winter Olympics next year. There will be some talk about the G8 meeting in Fermanagh but the real business is Syria and on that they are far apart.
This week the UK circulated a document suggesting options to lift the EU arms embargo in order to supply weapons to the Syrian rebels.
At the same time Russia is arming the Assad regime and there’s even talk of selling them sophisticated anti aircraft missiles. Mr Cameron and President Putin seem to be fast heading in opposite directions on Syria.
There may be a glimmer of hope however in the prospect of a possible peace conference agreed between the US and Russia in the last few days. But even here there is division on whether the end of the Assad regime can be a precondition to talks.
So the Prime Minister has his work cut out in sunny Sochi to come away with anything that looks like a meeting of minds.
The UK Government's arguments in favour of easing the EU arms embargo when the sanctions expire at the end of the month have been leaked in a document.
The British discussion paper sets out two options:
- Lift the arms embargo against the Syrian National Coalition completely
- Remove the words "non-lethal" from a list of exemptions, clearing the way for weapons to be sent to the country
– The Prime Minister's spokesman
The rationale behind exploring all the options, which is what we have long been saying with regard to the arms embargo, is the fact that we need to continue to, as part of pushing for a political solution, increase the pressure on the Assad regime. That's why we think that is the right thing to do.
Prime Minister David Cameron visits Russia this week to hold talks with Russian president Vladimir Putin over the need for diplomatic progress to end the conflict in Syria.
It comes as the UK steps up efforts to end a European Union arms embargo to enable the supply of weapons to forces opposed to Bashar Assad's regime.
A spokesman for the Prime Minister said Russia was an "important player" in discussions about the conflict in Syria.
He added that Mr Cameron will be "pushing for a political solution" and stressing "the urgent need to start proper negotiations and to force a political transition to bring the conflict to an end".