By Sami Aboudi and Yara Bayoumy
DOHA (Reuters) - To applause from Arab heads of state, a foe of Bashar al-Assad took Syria's vacant seat at an Arab summit on Tuesday, deepening the Syrian president's diplomatic isolation and diverting attention from opposition rifts.
Speaking at an annual gathering of Arab heads of state in the Gulf state of Qatar, Moaz Alkhatib said he had asked U.S. Secretary of State John Kerry for U.S. forces to help defend rebel-controlled northern parts of Syria with Patriot surface-to-air missiles. NATO swiftly rebuffed the idea.
The insurgents have few weapons to counter Assad's helicopter gunships and warplanes.
"It was a historic meeting. You could feel the grandiose nature of the meeting," said opposition spokesman Yaser Tabbara.
"It's a first step towards acquiring full legal legitimacy."
Alkhatib said the United States should play a bigger role in helping end the two-year-old conflict in Syria, blaming Assad's government for what he called its refusal to solve the crisis.
"I have asked Mr Kerry to extend the umbrella of the Patriot missiles to cover the Syrian north and he promised to study the subject," Alkhatib said, referring to NATO Patriot missile batteries sent to Turkey last year to protect Turkish airspace.
"We are still waiting for a decision from NATO to protect people's lives, not to fight but to protect lives," he said.
Responding to Alkhatib's remarks, an official of the Western military alliance at its headquarters in Brussels said: "NATO has no intention to intervene militarily in Syria.
"NATO calls for an end to violence in Syria, which represents a serious threat to stability and security in the region. We fully support the efforts of the international community to find a peaceful solution," the official said.
Michael Stephens, a researcher based in Qatar for Britain's Royal United Services Institute, said acceding to Alkhatib's request would effectively put NATO at war with Damascus.
NATO's current deployment of three Patriot missile batteries, in eastern Turkey, is intended to be purely defensive, shielding Turkey from possible attack from Syria. The Patriots are designed to shoot down hostile missiles in mid-air.
Alkhatib, a Sunni Muslim cleric, took Syria's chair at the summit for the first time despite announcing on Sunday that he would step down as leader of the Syrian National Coalition.
The emir of Qatar, a strong supporter of the struggle to topple Assad, asked his fellow-Arab leaders to invite the coalition delegation to represent Syria formally at the summit, despite the internal divisions plaguing the opposition.
The Arab League suspended Syria in November 2011 in protest at its use of violence against civilians to quell dissent.
In his opening speech, Qatar's Sheikh Hamad bin Khalifa al-Thani urged the U.N. Security Council to stop the "oppression and repression of the people" in Syria, halt the bloodshed and "present those responsible for these crimes against their people to international justice".
The United Nations says about 70,000 people have been killed in a conflict that began with peaceful anti-Assad protests and turned into an increasingly sectarian armed insurrection.
The war in Syria has divided world powers, paralysing action at the Security Council. The Arab world is also split, with Saudi Arabia and Qatar the most fervent foes of Assad, and Iraq, Algeria and Lebanon the most resistant to calls for his removal.
Syrian rebels again fired mortar rounds into central Damascus on Tuesday. State television said several people had been wounded by "terrorist" mortar bombs that landed in the Syrian Arab News Agency SANA compound in the Baramkeh district.
The attack followed a similar flurry of rebel mortar bombs that struck near the Opera House on Ummayad Square in the heart of Damascus, killing two people on Monday.
Syrian state TV did not cover the Arab League meeting in Qatar, airing a programme on makeup for women instead.
Alkhatib's decision to quit, which he blamed on the world's failure to back the armed revolt against Assad also appeared to be motivated by internal disputes in the alliance. It undermined the alliance's claim to provide a coherent alternative to Assad.
Liberals saw it as a protest against what they view as the rising influence of hardline Islamists in the Qatari-backed umbrella group set up in Doha in November to replace the ineffectual Syrian National Council.
Syria's Muslim Brotherhood, criticised for its grip on the Syrian National Council, appears to be wield as much sway on its successor coalition, which has won wide international backing, but has failed to shake an image as consisting mostly of foreign-backed exiles immersed in political infighting.
Jane Kinninmont, of Britain's Chatham House think tank, said Qatar and the other Gulf states had been frustrated that the United States in particular and also European powers had not done more to help the Syrian opposition.
"The Gulf countries contrast this to the Iraq war which many of them were quite dubious about, and they see a U.S. that's far less interventionist today, even though there's a much greater case for and immediate humanitarian need than there was in the case of Iraq."
(For an interactive timeline on Syria, please click on http://link.reuters.com/rut37s)
(Graphic on Damascus battle http://link.reuters.com/qat86t)
(Additional reporting by Mirna Sleiman and William Maclean, Omar Fahmy in Cairo, Oliver Holmes and Erika Solomon in Beirut and Adrian Croft in Brussels; Writing by Alistair Lyon)