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 Syrias 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.
