US President Barack Obama meets Saudi King Abdullah Friday as mistrust fuelled by differences over Iran and Syria overshadows a decades-long alliance between their countries.
Obama, who is due to arrive in Saudi Arabia late in the afternoon on a flight from Italy, is expected to hold evening talks with the monarch on a royal estate outside Riyadh.
Saudi Arabia has strong reservations about efforts by Washington and other major world powers to negotiate a deal with Iran on its nuclear programme.
It is also disappointed over Obama's 11th-hour decision last year not to take military action against the Syrian regime over chemical weapons attacks.
Saudi analyst Abdel Aziz al-Sagr, who heads the Gulf Research Centre, said Saudi-US relations are "tense due to Washington's stances" on the Middle East, especially Iran.
The recent rapprochement between Tehran and Washington "must not take place at the expense of relations with Riyadh," Sagr told AFP.
Sunni Muslim Saudi Arabia, long wary of Shiite Iran's regional ambitions, views a November deal between world powers and Iran over the latter's nuclear programme as a risky venture that could embolden Tehran.
The interim agreement curbs Iran's controversial nuclear activities in exchange for limited sanctions relief, and is aimed at buying time to negotiate a comprehensive accord.
But Sagr said "arming the Syrian opposition will top the agenda" during Obama's visit, his second since his election in 2009.
Analyst Khaled al-Dakhil spoke of "major differences" with Washington, adding that Obama will focus on easing "Saudi fears on Iran and on regional security."
Saudi Arabia, the largest power in the six-nation Gulf Cooperation Council, fears that a possible US withdrawal from the Middle East and a diplomatic overture towards Iran would further feed Tehran's regional ambitions.
Iranian-Saudi rivalry crystallised with the Syrian conflict: Tehran backs President Bashar al-Assad's regime, while several GCC states support the rebellion against him.
- 'Clearing the air' -
Obama's stances towards events reshaping the region "have strained (Saudi-US) relations but without causing a complete break," said Anwar Eshki, head of the Jeddah-based Middle East Centre for Strategic and Legal Studies.
US security and energy specialist professor Paul Sullivan said Obama meeting King Abdullah could "help clear the air on some misunderstandings."
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"However, I would be quite surprised if there were any major policy changes during this visit. This is also partly a reassurance visit," he added.
White House spokesman Jay Carney has said that "whatever differences we may have do not alter the fact that this is a very important and close partnership".
However, Riyadh seems to be reaching out more towards Asia, including China, in an apparent bid to rebalance its international relations.
Crown Prince Salman bin Abdulaziz visited China, Pakistan, Japan and India this month, reportedly to strengthen ties.
The US-Saudi relationship dates to the end of World War II and was founded on an agreement for Washington to defend the Gulf state in exchange for oil contracts.
OPEC kingpin Saudi Arabia is the world's top producer and exporter of oil.
Obama and the king are also expected to discuss deadlocked US-brokered Israeli-Palestinian peace talks.
They will also discuss Egypt, another bone of contention since the 2011 uprising that ousted Hosni Mubarak, who was a staunch US and Saudi ally.
The kingdom was dismayed by the partial freezing of US aid to Egypt after the army toppled Islamist president Mohamed Morsi last July -- a move hailed by Riyadh.
On Thursday, Egypt's Field Marshal Abdel Fattah al-Sisi resigned as defence minister after announcing he would stand for president.
Meanwhile, dozens of US lawmakers have urged Obama in a letter to publicly address Saudi Arabia's "systematic human rights violations," including efforts by women activists to challenge its ban on female drivers.
And rights group Amnesty International said Obama "must break the US administration's silence on Saudi Arabia's human rights record by taking a strong public stand against the systematic violations in the kingdom."
"It is crucial that President Obama sends a strong message to the government of Saudi Arabia that its gross human rights violations and systematic discrimination are unacceptable," said Hassiba Hadj Sahraoui, Amnesty's deputy director for the Middle East and North Africa.
"A failure to do so would undermine the human rights principles the USA purports to stand for," she added in a statement.
Amnesty also urged Obama to express "dismay" at the kingdom's ban on women driving as his visit coincides with a local campaign to end the globally unique ban.