US President Barack Obama will seek to ease Saudi concerns that he is neglecting an old ally when he visits Riyadh on Friday, months after top Saudis objected to what they saw as a growing rapprochement between Washington and their rival Iran.

Rifts over Middle East policy came to a head last year when Washington worked with other powers to ease sanctions on Iran in exchange for concessions on its disputed nuclear programme, and backed away from air strikes on Tehran's ally Syria.

Senior figures in Saudi Arabia warned in October and November there might be a "major shift" away from Washington and Riyadh might "go it alone".

Obama, on his first trip to the world's top oil producer since 2009, is expected to try to clear the air after the rare public spat, and to find common ground on Syria's civil war and the Israeli-Palestinian peace process, said officials.

"He wouldn't be going to Saudi Arabia, he wouldn't be seeing the king, if he didn't feel the need to reassure them so that they don't go off on tangents that will in effect... not only not be productive but could be destructive," said Dennis Ross, Obama's former top Middle East adviser.

Although the US is no longer a major importer of Saudi oil, Riyadh remains important to Washington for its cooperation in fighting Al Qaeda and its influence with other Arab states, particularly as Obama pushes for an extension of US-sponsored Israeli-Palestinian peace talks.

For Riyadh's ruling princes, Washington has always been the ultimate guarantor of security, using its military clout to contain regional threats.

"The two countries have clear differences, but these do not really affect them working together towards the peace and security of the region," said Abdullah Al-Askar, head of the foreign affairs committee in the Shoura Council, a body appointed by King Abdullah to advise on government policies.

However, the Saudis sharply faulted Obama for his response to the 2011 Arab uprisings, accusing him of discarding old allies who were ousted in revolts and of failing to stand up to their main regional rivals the Muslim Brotherhood and Iran.

In Syria, the Saudis see the civil war - pitting mostly Sunni rebels against President Bashar al-Assad, whose Alawite sect is an offshoot of Shi'ite Islam - as a pivotal battle in a wider struggle for Middle East influence with Iran.

Saudi Arabia has supported Syria's rebels while Tehran has backed Assad - echoing their support for opposing sides in other often sectarian conflicts in Iraq, Lebanon, Bahrain and Yemen.

SYRIA CRISIS

When Obama failed to bomb Assad's forces following a poison gas attack in Damascus in August the Saudis were furious, believing they had been misled about Washington's intentions. Their concerns were then compounded by the announcement in November that Washington had agreed a preliminary nuclear deal with Iran.

The White House defended Obama's policies, and some US officials privately likened the Saudi outbursts - all but unprecedented for a kingdom that prefers to conduct diplomacy in the shadows - to a temper tantrum that would have little lasting effect on bilateral ties.

But both sides now cast the president's visit as a chance to showcase a strong alliance.

"We will be reinforcing some of our most important relationships in the Middle East," Susan Rice, Obama's national security adviser, told reporters in Washington on Friday.

A senior Saudi official said bilateral ties between Riyadh and the United States were very robust, broad and institutionalised, adding that they shared many objectives and worked closely on an array of issues.

He described differences on Syria and Iran as being over tactics, rather than strategic objectives, and said Obama's visit was not about mending fences, but part of the regular high-level consultations between two leaders.

That sanguine take on the relationship may be based on a sense in Riyadh that its concerns over some of Obama's policies last year are fading, and a recognition that no other country can replace the American security umbrella in the Gulf.

Washington appears to be finding an accommodation with Egypt's military rulers, any big steps forward in the nuclear deal with Iran still seen a long way off and administration officials are increasingly vocal about their impatience with Assad.

Saudi leaders have hoped from early in the Syrian conflict for more US support for the rebels, which they see as crucial to transform them into a convincing enough force on the battlefield to push Assad's foreign backers towards a deal.

Gulf officials said there was now a sense that deeper US engagement in Syria's crisis may become possible with time, citing Washington's frustration at Assad's slowness in surrendering chemical weapons, even though the Obama administration has made clear it will not intervene militarily.

"There's no doubt that President Obama will face demands, requests, expectations from his Gulf counterparts to try and address that military balance in ways that could help resolve the conflict," Tamara Cofman Wittes, assistant Secretary of State for Near Eastern Affairs from 2009-12, told reporters on a recent conference call.

Wittes, director of the Saban Center for Middle East studies in Washington, said it would be hard for Obama to "get through this trip in an effective manor without having some clear decisions" about how far the United States could go in helping the Syrian opposition in coordination with Gulf allies. - Reuters