RIYADH - Syria's divided rebel and opposition groups are trying to forge a common stance to oust President Bashar al-Assad but the absence of prominent activists and a main Kurdish force from their talks in Riyadh shows that unity remains elusive.

Saudi Arabia, a strong supporter of rebels fighting for four years to topple Assad, is hosting the opposition this week in the most ambitious attempt yet to find an agreed platform ahead of talks with the government to end Syria's conflict.

Bringing the fragmented opposition together is seen by its backers as a crucial step to end a civil war which started with protests against Assad in 2011 and quickly drew in rival Sunni and Shi'ite Muslim powers across the Middle East.

Shi'ite Iran, Assad's main regional supporter, has criticized the meeting in the Sunni Muslim kingdom, saying it is designed to harm efforts to reach a peaceful solution to a war which has killed 250,000 people and displaced 12 million.

At a Riyadh hotel where the talks will start on Wednesday, security was stepped up and journalists were ejected as fighters and opposition leaders gathered. Special forces soldiers with body armor and assault rifles manned checkpoints.

An initial list of 65 invitees to the Riyadh talks has grown substantially, but critics say it still falls short of a fully inclusive meeting.(Reuters)