Arab and Western foreign ministers are to meet Syrian opposition officials in London in an effort to persuade them to attend the next round of peace talks,
A key group in Syria’s main opposition alliance is threatening to boycott the talks, expected next month in Geneva.
It says any deal must see President Bashar al-Assad step down, while Damascus says that is not on the table.
But the ministers will say opposition unity is vital if peace talks are to have any chance of success.
In London, foreign ministers from 11 countries – the so-called Friends of Syria group – will try to lay the groundwork for what is known as the Geneva II conference.
Britain, Egypt, France, Germany, Jordan, Italy, Qatar, Saudi Arabia, Turkey, the United Arab Emirates and the United States are also expected to reaffirm their view that the conference must be about a political transition in Syria away from the Assad regime.
Speaking ahead of the London meeting, US Secretary of State John Kerry stressed that Syria’s opposition would never agree to President Assad staying in power.
“If he (Assad) thinks he’s going to solve problems by running for re-election, I can say to him, I think that certainly this war will not end as long as that’s the case that he’s there,” Mr Kerry said after talks with Arab League officials in Paris.
“I don’t know anybody who believes the opposition will ever consent to Bashar al-Assad being part of the government. He has bombed and gassed people in his country… How can that man claim to rule under any legitimacy in the future?”
Meanwhile, President Assad reportedly told Lebanon’s al-Mayadeen television that he saw no reason why he could not stand for re-election in 2014.
“Personally, I don’t see any obstacles to being nominated to run in the next presidential elections,” he was quoted as saying by the channel.
On Monday, the main opposition alliance, the National Coalition, said it was postponing until early November meetings to decide whether to attend the Geneva conference.
The dominant group in the coalition, the Syrian National Council, is currently refusing to go.
The opposition has been further weakened by fighting between the moderate Free Syrian Army and Islamist rebel groups.
Western officials have been buoyed by the initial results of the chemical disarmament effort in Syria, the BBC’s diplomatic correspondent Jonathan Marcus says.
But he adds that they are painfully aware that the recent chemical deal has done nothing to alter the course of the civil war or to reduce the burgeoning humanitarian catastrophe in and around Syria.
Participants at the first round of talks in June 2012 (Geneva I) had sought to end the civil war by getting Damascus and the opposition to choose a transitional government.
More than 100,000 people have been killed since the Syrian conflict began in 2011.