BEIRUT - 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," Khatib said in a statement posted on his Facebook page.

The Syrian National Coalition has refused the resignation of its leader Ahmad Moaz al-Khatib, the opposition bloc said in a statement released hours after he announced he was quitting.

Neither the Coalition's presidential office nor its general assembly has accepted Khatib's resignation. "They are asking Mr Moaz al-Khatib to go back to his work as the president of the Coalition," the English-language statement said. "Khatib has led the Syrian National Coalition at a very critical stage. He has pushed the Coalition forward skilfully, and has gained popularity and acceptance among the Syrian people," it added.

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's 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.

Khatib's surprise resignation came just days after the first election in Istanbul of a rebel prime minister, 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," Khatib said. "I had made a promise to our great people that I would resign if any red lines were crossed."

He reportedly objected to the election of an interim premiership. An opposition source in Doha, where the Arab League is to hold a summit on Tuesday, told AFP that Khatib accused "certain countries, notably Qatar, of wanting to control the opposition" and of having imposed Hitto.

US Secretary of State John Kerry, reacting to the resignation while on a visit to Baghdad, said: "It is not a surprise ... It is almost inevitable in a transition of a group such as the opposition for these guys to change their place ...

"It is not about one person ... It is about an opposition that is bigger than one person and that opposition will continue," he told reporters.

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

"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," Muqdad said.

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.

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.

Hajji al-Bab, the No 2 of the Liwa al-Tawhid rebel brigade in Aleppo, said Khatib's resignation would make no difference on the ground.

The resignation "has no impact on the ground because we didn't vote for him. It concerns those who elected him, not the fighters," he told AFP, adding that some rebels had "not even heard of his election".

On Saturday, the FSA chief of staff, Selim Idriss, said "the Free Syrian Army, in all its groups and revolutionary entities, conditions its support and cooperation with a prime minister on consensus" among the opposition.

Hitto won an election in Istanbul after 35 out of 49 Coalition members voted for him following 14 hours of discussion in a closed meeting between key dissidents from inside Syria and abroad. But several key Coalition members walked out of the meeting and boycotted the vote.

And later some 12 Coalition members announced they had suspended their membership in the opposition body in protest against an election result they viewed as illegitimate.

In Doha, with the opposition fragmented, Arab foreign ministers meeting ahead of a summit on Tuesday made no announcement on who would take up Syria's seat, vacant since the League's suspension of Damascus in November 2011.

According to a League official who declined to be named, the decision has been left to the full summit. Before Khatib's announcement, the National Coalition's envoy to Doha, Nizar al-Haraki, said his group has has been invited to the summit, and that Khatib and Hitto were expected to arrive soon.

The Arab League on March 6 called on the coalition "to form an executive body to take up Syria's seat" and attend the summit, although Iraq and Algeria have expressed reservations, while Lebanon has distanced itself from the decision.