By Khaled Yacoub Oweis
AMMAN (Reuters) - 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.
"We will be meeting under the shadow of the advances of Nusra and other militants. The recent al Qaeda statements have injected a new urgency for the international community to push to end the conflict," said an official who will attend the meeting on the conflict that has killed more than 70,000 people.
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," another 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.
The Syrian National Coalition said in a statement on Sunday that the Nusra Front's announcement "contradicts the will of the Syrian people and the objectives of the revolution". But it added that the group could not be ignored.
Sectors of the opposition want to force a U.S.- and Russian-backed settlement on both Assad and hardline groups like the Nusra Front, but Russia has first to be won over, a senior opposition source said.
"There is a growing realization that a peaceful settlement has to be imposed on Assad and on the jihadists," the opposition source said.
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.
The Friends of Syria last met on February 28 in Rome.
Hardline Islamists including the Nusra Front and Ahrar al-Sham seized Raqqa in east Syria over the past two months. The Nusra Front also expanded its influence in the southern province of Deraa, according to opposition military sources.
Jordan's Prime Minister Abdullah Ensour told parliament on Sunday that Syrian crisis was "threatening the country's national security," adding that Jordan would appeal to the United Nations Security Council over the impact on its economy of hosting Syrian refugees.
Jordan says it has received Syrian 450,000 refugees.
At east one Lebanese man was killed on Sunday when a shell fired from Syria hit a built-up area in no man's land between the two countries, a Lebanese security source 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.
(Additional reporting by Suleiman al-Khalidi Oweis, Amman newsroom; Editing by Jason Webb)