When Saudi Arabia's veteran foreign minister, Prince Saud al-Faisal, made no annual address to the United Nations General Assembly last week for the first time ever, his unspoken message could hardly have been louder.



For most countries, refusing to give a scheduled speech would count as little more than a diplomatic slap on the wrist, but for staid Saudi Arabia, which prefers back-room politicking to the public arena, it was uncharacteristically forthright.



Engaged in what they see as a life-and-death struggle for the future of the Middle East with arch-rival Iran, Saudi rulers are furious that the international body has taken no action over Syria, where they and Tehran back opposing sides.



Unlike in years past, they are not only angry with permanent Security Council members China and Russia, however, but with the United States, which they believe has repeatedly let down its Arab friends with policies they see as both weak and naive.



Like Washington's other main Middle Eastern ally, Israel, the Saudis fear that President Barack Obama has in the process allowed mutual enemies to gain an upper hand.



The alliance between the US, the biggest economy and most powerful democracy, and Saudi Arabia, the Islamic monarchy that dominates oil supplies, is not about to break.



But, as happened 40 years ago when an Opec oil embargo punished US war support for Israel, Riyadh is willing - albeit without touching energy supplies - to defy Washington in defence of its regional interests. The two have been at odds over Egypt since the Arab Spring, and increasingly so on Syria, where Saudi Arabia could now do more to arm Sunni Muslim rebels.



The real focus of Saudi anger is the Shi'ite Muslim clerics who have preached Islamic revolution since coming to power in Tehran 34 years ago, and whose hands Riyadh sees orchestrating political foes in half a dozen Arab countries.



Already aghast at U.S. reluctance to back rebels fighting Syrian President Bashar al-Assad, Tehran's strongest Arab friend, Saudi princes were horrified to see Washington reaching out to Hassan Rouhani, the new Iranian president, last month.



"The Saudis' worst nightmare would be the administration striking a grand bargain with Iran," said former diplomat Robert Jordan, who was US ambassador to Riyadh from 2001 to 2003.



Although any meaningful US-Iranian rapprochement looks distant, Obama telephoned Rouhani, an emollient self-described moderate, during the United Nations General Assembly.



The Saudis now fear Obama may be tempted to thaw ties with Tehran by striking a deal to expand inspections of its atomic sites in return for allowing Iranian allies to go on dominating Arab countries such as Lebanon, Syria and Iraq. That such a bargain has never been publicly mooted from within the Obama administration has not stopped Saudis voicing their concerns.



"I am afraid in case there is something hidden," said Abdullah al-Askar, chairman of the foreign affairs committee in Saudi Arabia's advisory parliament, the Shoura Council. "If America and Iran reach an understanding it may be at the cost of the Arab world and the Gulf states, particularly Saudi Arabia."



Askar stressed he was not privy to government thinking on the issue and was speaking in a purely personal capacity.



The US has no interest in alienating the Saudis. But as it balances priorities, some friction may be unavoidable - friction that worries Washington less than before, as rising U.S. domestic oil output makes it less beholden to Riyadh.



In a rare appearance, King Abdullah, who is aged about 90, was shown on state television on Monday meeting Egypt's visiting military-backed interim head of state, Adly Mansour.



His words, carried prominently in Saudi media on Tuesday, were a forceful condemnation of the "terrorism, delusion and sedition" of Egypt's Muslim Brotherhood, which was ousted from government by the armed forces in July after street protests.



That even these comments conveyed a clear subtext of Saudi-U.S. friction showed how far the two allies' differences extend today on issues across the Middle East.



It was Obama's decision not to back Egyptian military strongman Hosni Mubarak against mass protests in early 2011 that first infuriated the Saudi royals, inspiring in them a sense of betrayal as they saw a mutual friend abandoned by Washington.



The American president's willingness to work with Mohamed Mursi, the Muslim Brotherhood figure elected Egypt's president last year, further vexed the Saudis, who see the organisation as a direct threat to their dynastic rule.



The kingdom was critical of US support for the rule of the modern Islamists of the Brotherhood in Egypt and privately contemptuous of Washington's qualms about July's military takeover in Cairo.



When US politicians suggested cutting aid to Egypt after Mursi was ousted, Riyadh offered to make up for any lost funds, undermining American Middle East policy and signalling it was prepared to go its own way. US aid may be cut but since July, Saudi Arabia has given $5 billion, three times as much as Washington's annual contribution.



"Usually the Saudis will not make any decision against US advice or interests. I think we're past this stage. If it isn't in our interests, we feel no necessity to bow to their wishes," said Mustafa Alani, an Iraqi analyst at the Gulf Research Centre in Jeddah with close ties to the Saudi security establishment.



Alani recited a litany of Saudi complaints, from Obama's failure to press Israel to end settlement building in the West Bank to his support for Arab Spring revolutions that toppled Egyptian and Tunisian autocrats who had supported US policy.



But, like many prominent Saudis, he was most concerned by Obama's approach to the crisis in Syria, a conflict viewed in the kingdom as a defining battle for Middle East supremacy between pro-Western leaders and non-Arab Iran.



For more than a year, Saudi officials have pleaded with Washington to enter the conflict, either with direct air strikes or the imposition of a no-fly zone, or by giving significant military aid to the mainly Sunni rebels.



When hundreds of civilians died in a chemical weapons attack in August, the Saudis were convinced this breach of a "red line" Obama had set last year would, finally, force tough US action.



The subsequent Russian-brokered deal to dismantle Assad's chemical weapons and thus avert air strikes was a bitter blow for Riyadh, whose main strategy in Syria had been to coax its American ally into the war.



That fractious backdrop explains the worry senior Saudis felt at Obama's flirtation with Rouhani in New York last month.



After years of counselling their closest foreign friends to beware Iran's rulers, describing them as "unstable", "meddling", "the head of the snake", in various diplomatic cables released by Wikileaks, they fear their advice is going unheeded.



For Saudi Arabia, the stakes seem high. Regional rivalry with Iran has played out with increasing venom.



Over the past decade the Saudis have watched with alarm as Iran's allies among the Arabs have grown in strength, dominating politics in Lebanon and Iraq and staging rebellions in Bahrain and Yemen in a move that seems to encircle Saudi Arabia.



They also fear Tehran is taking direct action against the kingdom itself: fomenting unrest among its Shi'ite minority; plotting to assassinate its envoy in Washington; and planting a spy ring in the country. Tehran has denied all those claims.



"If the US is really soft on Iran and letting Iran's friends win in Syria, what conclusions will Gulf governments draw?" one senior diplomat in the Gulf said some weeks ago.



The Saudis were concerned "about an eventual possible deal between Washington and Iran", he added, saying: "They will see what happens in Syria as a sign of that."



For Riyadh, Obama's acceptance of the UN chemical weapons deal with Damascus signalled the US had indeed gone soft.



"It's not about the principle of Iran negotiating with the US. We know this will happen sooner or later," said Alani.



"But it's coming at the wrong time, when this president has already been tested by the Iranians and Russians and proved he cannot stand up for his principles or protect our interests."



Former diplomats in Saudi Arabia argue that whatever the bumps in the road, that situation remains largely unchanged.



"The Saudis are very skilful politicians. They wouldn't be in power otherwise," said one who argued Washington would heed Saudi concerns. "Most American administrations, Republican or Democrat, listen to the Saudis and their advice on many things, including the Iranians - who they can trust and who they can't."



Yet the example of Saudi Arabia's divergence from the US line on Egypt suggests the kingdom may now be inclined to push against US policy in other areas, particularly on Syria.



Next Wednesday marks the 40th anniversary of the Arab oil embargo, when Gulf exporters blocked energy sales to penalise US support for Israel against the Arabs in the Yom Kippur War.



Today, the importance of steady oil supplies is one area where Washington and Riyadh are fully in sync, but Saudi Arabia might be prepared to push its own interests in other ways.



Several analysts and diplomats in the Gulf said the US had pressed Riyadh not to deliver "game-changing" weapons such as surface-to-air missiles to Syrian rebels, fearing they would fall into the hands of the Islamist militants.



Alani said that was one area where Saudi rulers may consider going their own way, particularly, he noted, as minor divisions within the family over how far to trust Obama had receded.



"There's not even one senior prince sympathising with him any more," he said. "They think he has lost his head."



How far such a move would alter the balance of the conflict in Syria is unclear. But it could spell a renegotiation of the US-Saudi relationship, one of the most enduring in the Middle East, said former US ambassador Jordan. The two may now become less willing to heed each other's concerns on other issues.-Reuters