The head of Syria's main opposition group has resigned, saying he had taken the step so he could work with more freedom.

Moaz Alkhatib, a former imam of the Umayyad Mosque in Damascus, was picked to head the National Coalition for Syrian Revolutionary and Opposition Forces in November after leaving Syria due to President Bashar al-Assad's crackdown on rebels.

"I had promised the great Syrian people and promised God that I would resign if matters reached some red lines," Mr 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.

A spokesman for Mr Alkhatib confirmed his resignation.

Last week, the coalition chose Western-educated former businessman Ghassan Hitto as a provisional prime minister to form a government to fill a power vacuum in Syria arising from the two-year-old revolt that has killed more than 70,000 people.

Earlier this year, Mr Alkhatib floated an initiative for the opposition to talk to Mr Assad's administration about a political transition, but said the Damascus government did not respond.

In his statement, Mr Alkhatib added: "We will follow the path with our brothers who aim for the freedom of our people."

He said official positions were "means to serve noble objectives" and not an aim in themselves.

Leaders of the coalition are due to attend an Arab League summit this week, where they will look for more support for their armed uprising.