By Angus McDowall
RIYADH (Reuters) - Saudi Arabia's effort to unite Syrian rebels in Riyadh next week will be a big test of its regional ambitions after years of bickering between opposition groups and serious misgivings about the initiative among major powers with a stake in the war.
Since King Salman took power in January, Riyadh has tried to position itself as leader of the Middle East's Sunni Muslims, most of whom want to see Syrian President Bashar al-Assad toppled and the influence of his Shi'ite ally, Iran, curbed.
Riyadh now sees an opportunity to shape the war in Syria after the Russian intervention, the European refugee crisis and the Paris attacks reawakened international engagement with the conflict and the threat posed by Islamic State.
"This conference is meant to change the situation on the ground," said a senior Western diplomat in the Gulf, pointing to the need to strengthen what he called the "moderate opposition" in Syria, which opposes both Assad and Islamic State.
Next week's gathering will be attended by around 65 members of the political and armed opposition, including around 15 representatives from armed groups. Of the two most powerful armed groups, Islamic State has not been invited and the al Qaeda offshoot Nusra Front is also not expected.
However, with Iran decrying the meeting as harmful to peace prospects, Turkey alarmed by the likely presence of Kurds, and Western countries concerned by the role that Islamists will play, Saudi Arabia may struggle to achieve that.
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 the rivalry's pivotal battlefield.
The war pits the Syrian army and allied militias including Lebanese Hezbollah fighters backed by Iran and Russia, against an array of competing rebel and jihadi fighters, who include Arabs and Kurds.
The rebel splits on the ground have been exacerbated by rival agendas of their supporters in the Gulf states, Turkey and Western countries. Syria's political opposition in exile also has little influence inside the country, further complicating efforts to form a united front against Assad.
While the rebels' external backers are united in describing Islamic State as a bitter enemy, Western countries see it as the biggest and most urgent threat, while Turkey and the Gulf states continue to regard Assad as the underlying problem.
Although Saudi Arabia last year helped bring together a group of Arab countries to support U.S.-led air strikes against Islamic State, its military role in the coalition was mostly symbolic.
While it has continued flying some missions against the group this year, say diplomats, its role has been greatly reduced as a result both of unease with the coalition's strategy and Riyadh's overwhelming military focus on the war in Yemen.
The most recent Saudi air strikes in Syria that the diplomat and a Saudi insider could remember came over a month ago, they said. A second senior Western diplomat in the Gulf said Saudi Arabia was still flying some missions.
Now Riyadh wants to focus on forging from the Syrian opposition a coherent body that can function as a serious interlocutor, and dismiss the argument made by Assad, Iran and Russia that it is dominated by militants.
Gaining some kind of agreement at the meeting between a wide variety of opposition groups will not be the only problem, however. A bigger task may be persuading Riyadh's allies to accept any outcome from the talks.
"Just to be organizing this is a big deal. There has been a lot of hedging over who to invite. It's an evolving discussion between the Saudis, the U.S., the Turks and others. There are lots of issues with various groups," said the second senior Western diplomat.
Turkey has clear reasons for concern. Ankara faces not only the threat from Islamic State, but also the prospect of Assad, shielded by Russia and Iran, holding on to power, while Kurdish rebels backed by the United States make territorial gains.
It has warned both the United States and Russia that it will not tolerate any such gains by the Washington-backed Kurdish YPG militia close to its frontiers in north-western Syria and privately deprecates Saudi efforts to include the group.
Western countries fret about powerful Islamist groups like Ahrar al-Sham, which are ideologically similar to al Qaeda but which Turkey and Gulf states view as moderate because they voice no ambitions to wage a wider regional jihad.
Renewed tensions between Turkey and Russia after the shooting down of a Russian jet present another obstacle to reaching a broader agreement on Syria, but Riyadh still regards engagement with Moscow as vital.
Deputy Crown Prince Mohammed bin Salman has visited Russia twice this year, and according to both Western diplomats and Gulf diplomats, still thinks there is a possibility that Moscow will eventually drop its support for Assad.
According to Mustafa Alani, an Iraqi security expert with close ties to Riyadh's Interior Ministry, it was because of Russia that Riyadh agreed to host the meeting, working to deliver a group that can negotiate opposite Assad.
"They understand the complexity of their task. How difficult it will be to come up with something tangible," he said. "So they will not aim to create a centralized leadership that reflects who controls the ground because this will be impossible. They will aim to produce a centralized leadership that has reasonable, rational thinking and can sit at the table."
(Additional reporting by Yara Bayoumy in Dubai and John Irish in Paris; Editing by Giles Elgood)