Plans for talks to end the fighting in Syria are in jeopardy as the opposition has refused to attend unless President Bashar al-Assad is forced from power.
Meanwhile, Saudi Arabia made clear it would no longer co-operate with the United States over the civil war.
Western nations and their Middle Eastern allies have pressed Syria's fractured opposition to join the proposed peace talks.
However, Mr Assad has indicated he will not bow to opposition demands that he should step down as a pre-condition.
The US and Russia said in May they would convene a Geneva 2 conference to try to end a conflict that has killed well over 100,000 people and forced millions from their homes, but it faces huge obstacles and no firm date has been set.
British Foreign Secretary William Hague, hosting a meeting of 11 nations in London, said it was vital that the western-backed Syrian opposition join the talks.
"We urge the National Coalition to commit itself fully and to lead and form the heart of any opposition delegation to Geneva," he told a news conference.
However, opposition factions are unwilling to discuss anything except the immediate departure of Mr Assad, who has said he sees no reason why he should not run for re-election next year.
"The Sultan must leave," said Syrian opposition chief AhmedJ arba in the text of a speech to the meeting, referring to Mr Assad.
"Geneva cannot succeed and we cannot take part if it allows Assad to gain more time to spill the blood of our people while the world looks on."
Many of the mostly Islamist rebels fighting in Syria refuse to recognise the exiled opposition favoured by the west.
Efforts to present a united front suffered a further setback when it emerged that Saudi Arabia's intelligence chief had said the kingdom would make a "major shift" in relations with the US in protest at its perceived inaction over Syria and its overtures to Iran.
Prince Bandar bin Sultan told European diplomats that the US had failed to act on Syria among other Middle Eastern issues, according to a source close to Saudi policy.
"The shift away from the US is a major one," the source said.
There would be no further coordination with the US over the war in Syria, where the Saudis have armed and financed rebel groups fighting Mr Assad, the source said.
Saudi anger boiled over after the US refrained from military strikes in response to a poison gas attack in Damascus in August when Mr Assad agreed to give up his chemical arsenal.
Saudi Arabia is also concerned about signs of a tentative reconciliation between the US and Iran, the Saudis' old enemy, which may be invited to Geneva.
US Secretary of State John Kerry met Mr Jarba, who leads the opposition Syrian National Coalition, before the London talks began, but there was no word on the outcome.
Mr Kerry said events may have moved in Mr Assad's favour since he and Russian Foreign Minister Sergei Lavrov announced plans for the peace conference five months ago, but that the aim remained to get both sides to choose a transitional government.
"I don't know anybody who believes that the opposition will ever consent to Bashar al-Assad being part of that government," Mr Kerry said.
"If he thinks he is going to solve problems by running for re-election, I can say to him ... this war will not end."
Mr Hague said no military solution existed and urged Syrians to "make the compromises necessary for a peace process to work".