BEIRUT (AFP) - A Syrian former premier who defected from President Bashar al-Assad's regime in 2012 is a frontrunner to head the main opposition National Coalition, a dissident told AFP Sunday.

"Riad Hijab and (current coalition chief) Ahmad Jarba are the two candidates for the presidency," coalition member and veteran dissident Samir Nashar said.

"The election is about to take place," he added, speaking to AFP by telephone from Istanbul.

Mr Hijab, the highest-ranking defector so far, abandoned Assad a year and a half into the revolt demanding the fall of his regime, fleeing via Jordan. He comes from the eastern province of Deir Ezzor, much of which has been destroyed in nearly three years of all-out war.

Mr Hijab is competing for the presidency against Mr Jarba, who became coalition chief after a heated election on July 6. Mr Jarba has close ties to Saudi Arabia, a key backer of the anti-Assad revolt.

The coalition holds an election every six months to choose its head.

The latest vote is taking place in the framework of the coalition's general assembly meeting in Istanbul, which opened on Sunday and is set to end on Monday.

Coalition members are also expected to take a final decision on whether to attend proposed peace talks in Switzerland slated for January 22.

A key coalition bloc, the Syrian National Council, has reaffirmed its decision to boycott the talks, raising fears the opposition will not attend the so-called Geneva 2 conference at all.