* Resignation comes ahead of an Arab League summit
* It could throw a spanner in the works, says analyst
* Kerry says resignation is no surprise
* Third reported use of chemical-type weapons in conflict (Adds activists reporting chemical weapons attack)
By Khaled Yacoub Oweis
AMMAN, March 24 (Reuters) - The head of Syria's main opposition group resigned on Sunday, weakening the moderate wing of the two-year revolt against President Bashar al-Assad's rule and complicating Western efforts to back the rebels.
The resignation of Moaz Alkhatib, a former imam of the Umayyad Mosque in Damascus who had offered Assad a negotiated exit, could make the West more cautious in supporting the revolt. Alkhatib was seen as a moderate bulwark against the rising influence of al-Qaeda linked jihadist forces.
Syrian opposition leaders are due to attend an Arab League summit this week, Qatar said earlier on Sunday, looking for more support for their armed uprising.
Michael Stephens, researcher at the Royal United Services Institute in Doha, said Alkhatib's resignation throws a spanner into the summit.
"The premise of the summit is to determine whether the opposition has a legitimate right to sit with Arab states," Stephens said. "While Khatib may have blamed the EU summit, it is well known that the Arab League is meeting today, and his resignation will have a serious effect on the process."
Alkhatib was picked to head the Western and Gulf-backed National Coalition for Syrian Revolutionary and Opposition Forces, which was formed in Qatar in November.
His resignation is seen as having been to some degree caused by Qatar, the main backer of his political foes in the coalition, and the country spearheading Arab support for the revolt as its geopolitical ramifications deepen.
The conflict pits Syria's Sunni Muslim majority against Assad's Alawite sect, an offshoot of Shi'ite Islam that has controlled the country for almost five decades, deepening the Sunni-Shi'ite divide in the Middle East and raising tension between Gulf states and Iran.
Asked to comment on Alkhatib's resignation, Qatari Foreign Minister Sheikh Hamad bin Jassim al-Thani said in Doha: "We are very sorry for this, and I hope he reviews his resignation."
PROMISE TO GOD
Alkhatib quit after the coalition berated him for offering Assad a deal and after the group went ahead, despite his objections, with steps to form a provisional government that would have further diminished his authority.
"I had promised the great Syrian people and promised God that I would resign if matters reached some red lines," Alkhatib said in a statement on his official Facebook page, without explaining exactly what had prompted his resignation.
"Now I am fulfilling my promise and announcing my resignation from the National Coalition in order to be able to work with freedom that cannot be available within the official institutions," he said.
U.S. Secretary John Kerry, on a trip to Baghdad, expressed regret at Alhatib's decision.
"With respect to Moaz Alkhatib, I am personally sorry to see him go because I like him on a personal level and because I have appreciated his leadership but the notion that he might resign has frankly been expressed by him on many different occasions in many different places and it is not a surprise," Kerry said.
He made an unannounced visit to Iraq on Sunday and said he told Prime Minister Nuri al-Maliki of his concern about Iranian flights over Iraq carrying arms to Syria.
"Anything that supports President Assad is problematic," Kerry told reporters.
RISE OF ISLAMISTS
Last week, the coalition chose Islamist-leaning technocrat Ghassan Hitto as a provisional prime minister to form a government to fill a power vacuum in Syria arising from the revolt that has killed more than 70,000 people.
Hitto visited the Syrian commercial hub of Aleppo on Sunday to draw up a plan to restore services in parts of the city that have fallen to the opposition, according to a statement issued by his office.
Alkhatib, who had argued insufficient groundwork had been done to start forming a government, was weakened considerably, along with a moderate wing of the revolution as jihadist Salafists play a bigger role on the battlefield.
The rise of Salafists as the most effective fighting force, and their recent gains on the ground, have contributed to the coalition adopting a more hardline stance in recent weeks, rejecting dialogue with Assad except under strict conditions and ignoring promises to include more women and minorities.
Hitto, whose cabinet is supposed to govern rebel-held areas currently ruled by hundreds of brigades and emerging warlords, was backed by the Muslim Brotherhood and coalition Secretary General Mustafa Sabbagh, who has strong links with Qatar.
"Basically Qatar and the Brotherhood forced Alkhatib out. In Alkhatib they had a figure who was gaining popularity inside Syria but he acted too independently for their taste," said Fawaz Tello, an independent opposition campaigner.
"They brought in Hitto. The position of Alkhatib as leader became untenable."
The appointment of Hitto prompted nine people to suspend their membership in the 62-member body, saying that promises to reform the coalition and respect consensus have been discarded.
Earlier this year, Alkhatib floated an initiative for the opposition to talk to Assad's administration about a political transition, but said the Damascus government did not respond.
Moaz al-Shami, a leading activist in Damascus, said Alkhatib's resignation deprived the coalition, which consists mostly of exiles, of the figure best-known inside Syria, but that Alkhatib still could still play a major role in the revolt.
Alawite opposition activists called for Assad's overthrow on Sunday and urged their co-religionists in the army to rebel.
In the first meeting of its kind by Alawites who support the revolt, delegates distanced themselves from Assad.
"We call on our brothers in the Syrian army, specifically members of our sect, not to take up arms against their people and to refuse to join the army," the delegates said in a statement after two days of meeting in Cairo.
In the town of Adra on the outskirts of Damascus, opposition campaigners said Syrian government forces fired chemical weapons from multiple rocket launchers at rebels surrounding an army base, killing two fighters and wounding 23.
There was no independent confirmation of the attack. Video footage showed one man in a hospital bed with his hands shaking, while doctors were trying to stabilise another man. Another patient was shown with saliva pouring from his mouth. (Addtional reporting by Omar Fahmy and Tom Perry and Ulf Laessing in Cairo, Arshad Mohammed in Baghdad, Sami Abboudi, Bill Mclean and Regan Doherty in the Gulf; Editing by Stephen Powell)