British Foreign Secretary William Hague has urged the Syrian opposition to commit itself to a planned peace conference in Geneva next month.
After hosting talks in London between high-level Western and Arab officials, and Syrian opposition figures, Mr Hague said all participants were agreed that President Bashar al-Assad could play no role in any future government.
This view was echoed by the American Secretary of State, John Kerry.
The BBC reports that Mr Hague said the talks offered Syrians the "best hope to improve their lives".
Mr Hague's US counterpart Mr Kerry said he believed and hoped the meeting would go ahead as planned but that Western countries could not control whether all parties would attend.
A key group in Syria's main opposition alliance has threatened to boycott next month's talks.
Head of the main opposition grouping, Ahmad Jarba, the Syrian National Coalition (SNC), said the coalition would decide at a meeting next week whether to participate.
The Friends of Syria group of countries which is brokering the "Geneva II" talks envisages the establishment of a transitional governing body with full executive powers and a political transition away from the Bashar al-Assad regime.