Syrian political and armed opposition groups meeting in Riyadh have agreed on a framework for negotiations with Bashar al-Assad, opposition sources told AFP on Thursday.

"An agreement has been reached on... a unified vision for the settlement process and a supreme committee that would act as a reference for the negotiating team, the composition of which will be specified later," said Suhair Atassi, a member of the National Coalition, the main opposition grouping.

However earlier on in the day, one of Syria's main rebel groups, Ahrar al-Sham, pulled out of opposition talks aimed at forging a united front ahead of potential discussions with Bashar al-Assad's regime.

It said it took the decision because of "the fundamental role... given to personalities linked to the regime" at the conference in Riyadh.Dozens of Syrian opposition groups and factions gathered in a meeting held at Riyadh, the capital of the staunch ally of the groups, Saudi Arabia, to forge a unified front against Syria's Bashar Assad, whose fate is going to be discussed by world powers in Vienna in January.Most of the main opposition groups participated in peace talks for the first time since the beginning of the almost five-year-long crisis and getting rid of Assad, who has been given a new lease of life by Russian and Iranian intervention, is crucial for the groups.The most salient group among the participants, Ahrar al-Sham, said on Wednesday that the meeting must insist that Assad face justice and that Syria's "institutions of oppression" are dismantled. Together with Ahrar al-Sham, there are Western-backed opposition groups, the Syrian National Coalition, and the Syria-based National Coordination Body, as well as groups like the Western-backed Free Syrian Army and the Saudi-backed Jaysh al-Islam.It is reported that Al-Qaida's affiliate in Syria, the al-Nusra Front, and the PKK's Syrian-affiliate, the People's Protection Units (YPG), have not been invited to Riyadh. The Syrian Democratic Forces, a recently formed alliance of the YPG and smaller Arab and Christian militias was also not invited.The main Western-backed political alliance, the National Coalition for Syrian Revolutionary and Opposition Forces, which was created in Doha in November 2012, will send 20 delegates to Riyadh. Recognized by 120 countries as the representative of the Syrian people, it participated in two rounds of peace talks in Geneva with regime delegates in 2013 and 2014. The coalition supports the implementation of the 2012 Geneva Communique, which calls for the establishment of a transitional governing body in Syria. The Turkey-based group led by Khaled Khoja insists that Assad must go.