A Syrian opposition leader, taking Syria's seat at an Arab summit for the first time today, said the US should use Patriot missiles to protect rebel-held areas from President Bashar al-Assad's airpower.

Moaz Alkhatib said he had asked US Secretary of State John Kerry for US forces to help defend rebel-controlled northern parts of Syria with Patriot surface-to-air missiles.

The insurgents have few weapons to counter Mr Assad's helicopter gunships and warplanes.

Mr Alkhatib said the US should play a bigger role in helping end the two-year-old conflict in Syria.

He blamed Mr 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," Mr 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.

The Sunni Muslim cleric took over Syria's vacant chair at the Arab League summit in Doha 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 Mr 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 UN 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 UN 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.

Mr Alkhatib's decision to quit, which he blamed on the world's failure to back the armed revolt against Mr Assad, also appeared to be motivated by internal disputes in the alliance.

It undermined the alliance's claim to provide a coherent alternative to Mr Assad.