* Riyadh, Washington disagree on weapons for rebels
* Saudi Arabia sees Syria war as pivotal Mid-East struggle
* Washington worried about involvement in messy conflict
* Saudi concerns ultimately directed at main rival Iran
By Yara Bayoumy and Angus McDowall
DUBAI/RIYADH, May 5 (Reuters) - Differences between the United States and Saudi Arabia over Middle East policy persist, despite attempts to shore up their old alliance, and may prove calamitous for Syrian rebels.
Although there is evidence that some American weapons are starting to find their way to more moderate groups fighting Syrian President Bashar al-Assad, disagreements over what to supply, and to whom, have hindered the fight.
Rebels lament a lack of anti-aircraft missiles to help counter Assad's air force.
Saudi Arabia and Qatar have been funding the rebels for years now, arguing that the war in Syria is a battle for the future of the Middle East, pitting pro-Western forces against Riyadh's main enemy Iran and Islamist militants.
However, while the administration of U.S. President Barack Obama also blames Assad for the violence and wants him to leave power, it sees the conflict very differently.
American officials fear involvement in a messy civil war for which they see no clear military solution and which threatens to radicalise a new generation of Islamists who hate the West.
Among the rebels, the failure of the Saudis and the Americans to cooperate better stirs disillusion. Two hours of talks between Obama and Saudi King Abdullah in March appear to have done little to alter that sentiment.
"If the Americans refuse to give us anti-aircraft (missiles), for example, why doesn't Saudi give it to us?" a Syrian rebel commander in Aleppo whose brigade fights alongside the extremist al-Nusra Front told Reuters by Skype.
In private, American and Saudi officials defend a relationship that in many ways remains strong and broad-based.
But they acknowledge a fundamental divergence over how to approach big political conflicts in the Middle East that were aggravated by the Arab spring, particularly what Riyadh sees as Iranian expansionism across the region.
When Washington agreed a preliminary deal with Tehran in November over its nuclear programme, Riyadh feared it would reduce political pressure on Iran, giving it more scope to push its interests across the region.
The Saudis were also angry when Obama did not do more to back Egypt's Hosni Mubarak who was forced from power in 2011, and when Washington criticised the army for ousting his successor, Mohamed Mursi of the Muslim Brotherhood.
Meanwhile Assad appears to be gaining ground and has told a Russian official the heavy fighting will be over within a year.
"I'm afraid that what remains of the Syrian state will vanish, so I would say the United States has had a big failure in this regard," said Abdullah al-Askar, chairman of the foreign affairs committee for Saudi Arabia's appointed Shoura Council, which advises the government.
But if the Saudis felt stymied, so did the Americans.
"The frustration with the Saudis was that they never gave us a plan," a former senior U.S. diplomat who worked in the region told Reuters.
The former diplomat said there had to be a strategy that included pulling the opposition together into a political and military union dominated by moderates, while arm-twisting Assad's main backers in the Security Council: Russia and China.
"There's got to be something more than throwing weapons and suitcases of money," the former diplomat said.
Riyadh's main Syria strategy has been based on persuading Washington of the need to bring its far greater diplomatic, military and planning clout to bear in helping the rebellion.
"We want the Americans to use their Tomahawks and F16s and beat the hell out of Bashar al-Assad. But at the same time I can see the Americans saying to the Saudis 'You guys have F15s too'," said Jamal Khashoggi, head of a Saudi television news channel owned by a nephew of King Abdullah.
The United States fears that any heavy weapons or training for the rebels might leak to militants who would then turn on the West, repeating the U.S. experience in 1980s Afghanistan.
While Riyadh is aware of the danger of militant blowback - as happened a decade ago with an al Qaeda campaign of attacks in the kingdom - it sees U.S. reluctance as a strategic error.
Saudi officials think the failure to back moderate rebel groups earlier not only encouraged Assad, but allowed militants to emerge as the strongest element in the opposition.
Although Saudi authorities repeatedly announced that donations to Syrian rebel or humanitarian groups should only go via official channels to ensure they did not end up in militant hands, some private donations were likely made to radicals.
Officials in the kingdom were frustrated at what they saw as American dithering, particularly after Obama backed down from a strike on Syria following a chemical weapons attack in the Damascus suburbs in August.
Still, cooperation has improved in recent months, American officials believe, and U.S.-made rockets have started to appear on the battlefield.
One reason for the better atmosphere between the allies may be the departure from office this year of intelligence chief, Prince Bandar bin Sultan, who was running Saudi policy on Syria.
His abrasive manner and sometimes erratic way of working caused friction with the Americans.
However, Washington still wants more openness on the Saudi side, said a diplomatic source in the Gulf, one of several interviewed for this article.
But the source added that the Saudis still felt left out on a limb by last year's non-strike.
"They see brush fires all around them and are concerned Washington is not doing more to help the Syrian opposition," the diplomatic source said.
For all its differences with Riyadh, Washington remains the kingdom's most important ally, sharing an outlook that values regional stability and a tough approach to Islamist militants.
A big American military presence in the Gulf still protects Saudi borders from foreign enemies, the kingdom's armed forces are equipped mostly with American hardware and a web of personal relations binds officials, diplomats and businessmen.
But after the Arab Spring destabilised one of Saudi Arabia's neighbours after another, Riyadh's perception of a pivotal threat was not matched in Washington.
ROOT OF UNEASE
The former U.S. diplomat said this still influenced Riyadh's view of Washington's nuclear talks with Iran, despite attempts by Obama and other officials to assuage Saudi fears.
Gary Grappo, a former deputy chief of mission in Riyadh, said the Saudis were intensely suspicious of Iran.
"There was an overwhelming obsession with Iran and the threat that it posed. We heard from Saudi officials, some quite senior, that Iran's intention is to position itself as leader of the Muslim world, especially after the Shi'ites re-established control over their holy sites in Iraq - Kerbala and Najaf."
"It sounds like an exaggeration to us, but I heard it: 'The next destination is Mecca and Medina'," said Grappo.
Though frayed, the alliance is unlikely to break, with the former diplomat describing U.S.-Saudi relations in terms of a longstanding marriage:
"It's like a couple that's been married for 40 years - you can't imagine not being together, but you can't seem to avoid poking each other." (Additional reporting by Mariam Karouny in Beirut; Editing by William Maclean and Giles Elgood)