BEIRUT (AFP) - Syria's opposition chief Ahmed Moaz al-Khatib on Sunday announced his resignation from the National Coalition, throwing Syria's divided opposition into disarray ahead of an Arab summit.

"I announce my resignation from the National Coalition, so that I can work with a freedom that cannot possibly be had in an official institution," Mr Khatib said in a statement posted on his Facebook page. The resignation threw Syria's divided opposition into chaos just two days before Arab heads of state were due to decide whether to give it Damascus' vacant seat in the Arab League.

Set up in Doha in November, the coalition is a dissident group recognised by dozens of states and organisations as legitimate representative of the Syrian people.

Mr Khatib's surprise resignation came just days after the first election in Istanbul of a rebel prime minister, Mr Ghassan Hitto, and just over two years on from the outbreak of a popular revolt against President Bashar al-Assad.

"For the past two years, we have been slaughtered by an unprecedentedly vicious regime, while the world has looked on," Mr Khatib said. "I had made a promise to our great people that I would resign if any red lines were crossed." He had reportedly wanted to quit for some time, objecting to an interim premiership.

An opposition source in Doha, where the Arab League is set to hold a summit on Tuesday, told AFP that Mr Khatib accused "certain countries, notably Qatar, of wanting to control the opposition" and of having imposed Mr Hitto.

US Secretary of State John Kerry, meanwhile, on a visit to Baghdad, told reporters that the resignation was "not a surprise... It is almost inevitable in a transition".

Coinciding with the resignation, an official in the rebel Free Syrian Army (FSA) which has been fighting the regime for the past two years told AFP it does not recognise Mr Hitto as rebel prime minister.

"We in the Free Syrian Army do not recognise Ghassan Hitto as prime minister," said FSA political and media coordinator Louay Muqdad. "I speak on behalf of the (rebel) military councils and the chief of staff when I say that we cannot recognise a prime minister who was forced on the National Coalition, rather than chosen by consensus."

Mr Hitto himself on Sunday, after having pledged to set up a government based inside Syria, visited the northern province of Aleppo, large swathes of which are under rebel control, according to the Syrian interim government's Facebook page.

Mr Hitto "held a two-hour meeting with a delegation bringing together (anti-regime) Aleppo province local council officials and representatives of the judicial council", said the statement.