By Angus McDowall
RIYADH (Reuters) - 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.
The Kurdish administration that runs swathes of north Syria was not invited. Rebels in western Syria do not trust the main Kurdish militia, the YPG, because they say it cooperates with Damascus rather than fighting it.
"It is not all-encompassing. It is not the consolidated, overall opposition platform," a Western diplomat following Syria said of the Saudi meeting. "I do not expect Riyadh to be a constructive step ... The whole thing has been very acrimonious, and it looks like a Saudi-Turkish wish-list."
Alongside Saudi Arabia, Turkey is one of the main foreign backers of the rebellion against Assad.
Syrian Kurdish, Arab and Turkmen groups held their own opposition conference in the Kurdish-controlled town of al-Malikiya at the Syrian-Iraqi border on Tuesday.
Haytham Manna, an activist in exile, said he would not attend the Riyadh meeting because it included "people who support an Islamic emirate".
With Iran decrying the meeting as harmful to peace prospects and Western countries concerned by the role that Islamists will play, Saudi Arabia may struggle to unite the enemies of Assad.
For Saudi Arabia, Syria has been secondary to Yemen this year as the main cockpit in an overarching struggle for regional influence with Iran, but the ruling Al Saud continue to regard the Syrian civil war as a pivotal battlefield in the rivalry.
Fighting has escalated in Syria in recent weeks and Russian warplanes have intervened to support Damascus while a U.S.-led coalition has stepped up strikes against Islamic State targets from the crowded skies over the country.
BLOODSHED, REFUGEES AND ATTACKS
Increased bloodshed, an influx of Syrian refugees into Europe, and a wave of international attacks claimed by Islamic State revived international efforts to contain the violence.
"This is the first meeting where we have all the opposition, the politicians and the armed groups," said Hadi al-Bahra, a member of the Turkey-based SNC opposition coalition.
Those talks should start by Jan. 1, under the terms of an agreement reached by world powers and regional states at a meeting in Vienna a month ago.
Participants invited to the Riyadh meeting include Islamist factions Islam Army and Ahrar al-Sham, a group whose founders had links to al Qaeda. Ahrar al-Sham fights alongside the Nusra Front, al Qaeda's Syrian wing, while espousing a nationalist agenda.
A dozen rebel groups who fight under the banner of the Free Syrian Army are also due to attend, including groups vetted by the United States that have received foreign military aid.
Of the two most powerful armed groups in Syria, Islamic State has not been invited and the al Qaeda offshoot Nusra Front is also not expected.
National Coalition member Nagham al-Ghadri said the two-day meeting aimed to agree on a document to take to any talks with the Damascus government, as well as an agreed negotiating team.
She said the opposition would not back down from its demand that Assad step down as soon as a transitional ruling body - which an international meeting on Syria called for three years ago - is established.
"The minute the transitional period should start, he should leave. We don't agree that he could stay during the transitional period," Ghadri said.
The Syrian government has dismissed any talk of a transitional body being imposed in Syria, saying any change in power in Damascus must be decided by the Syrian people.
Some Western countries which called for Assad to step aside in 2011 have softened their demands, suggesting he could at least remain for an interim period.
Ghadri said the outside world was sending mixed messages over Syria, showing that international divisions ran even deeper than any splits in the Syrian opposition.
"It's not just us. We know what we want," she said. "Some countries don't know what they want from Syria."
(Additional reporting by Tom Perry and Sylvia Westall in Beirut, and Suleiman al-Khalid in Amman; Writing by Dominic Evans, editing by Sami Aboudi and Peter Millership)