International powers will search for a peaceful settlement to Syria's civil war with fresh urgency at an Istanbul meeting after a rebel faction aligned itself with al Qaeda, diplomats and opposition sources said on Sunday.
Saturday's meeting of 11 countries from the Friends of Syria alliance will come after the al-Nusra Front, among the strongest formations seeking to topple President Bashar al-Assad, pledged allegiance to al Qaeda leader Ayman al-Zawahri on April 10.
Western powers, which want to see the end of the Assad family's 43-year rule but do not want to intervene militarily in Syria, have been alarmed by the advance of groups like the Nusra Front in a conflict which has deepened the Middle East's sectarian divide.
Among those invited to Istanbul will be Moaz Alkhatib, a moderate cleric from Damascus, who said he was resigning as head of the Syrian National Coalition in March after other members of the main opposition group attacked his proposal for negotiating with Assad, the sources said.
Envoys representing most of the 11 Friends of Syria countries met in Cairo this month to press Alkhatib to stay on as leader of Coalition, a 60-member opposition umbrella group backed by the West and Gulf Arab states, the sources said.
The Istanbul meeting, whose full agenda has not been finalized, will also discuss how to pressure Assad, who has been backed by Iran and Russia, into accepting a negotiated settlement, the diplomats said.
"The international powers are inclined to call Assad's bluff and see if he is ready to accept a peaceful solution," a diplomat said, adding that Russia might also favor such a move.
The conflict, which pits the Sunni Muslim majority against Assad's supporters among his Alawite sect, has prompted both Sunni and Shi'ite militants from elsewhere in the Middle East to fight in Syria. The Alawite sect is an offshoot of Shi'ite Islam.
In February, Syrian Foreign Minister Walid al-Moualem said in Moscow that Assad's government was ready for talks with rebels. But fighting intensified and rebels made gains particularly in Syria's east, which accounts for all the country's oil output and most of its grains production.
The Istanbul meeting will be attended by representatives from Turkey, Egypt, Jordan, United Arab Emirates as well as Qatar, and Saudi Arabia - the two main Arab powers backing the two year revolt. From the West, the United States, Britain, Germany, Italy and France will attend, the sources said.
Witnesses say fighting intensified in the last few days in the Qusair region along the Syrian-Lebanese border in the Bekaa Valley between Syrian Shi'ite militia backed by Assad's ally Hezbollah and Sunni rebel fighters. The conflict has killed more than 70,000 people.