Saudi Arabia's King Salman will meet US President Barack Obama in Washington on Friday, aiming to push him for more support in Riyadh's efforts to counter Iran after it agreed to a nuclear deal that will relieve Tehran of some international pressure.

Despite Saudi disappointment with Obama's push for a nuclear deal with Iran and his lack of direct action against President Bashar Al-Assad in Syria, the US role in Riyadh's war in Yemen shows Washington remains the kingdom's core strategic partner.

"The relationship is entering a new phase. It is still a partnership, but Saudi Arabia is becoming more independent. And I think the Americans like that. They are not objecting to our active foreign policy, but cooperating with us on that front," said Jamal Khashoggi, head of Al-Arab News Channel, owned by Saudi Prince Alwaleed bin Talal.

The relationship, a mainstay of the Middle East's security balance, has suffered turbulence since Riyadh faulted what it saw as Obama's withdrawal from the region, and a perceived tilt toward Iran since the 2011 Arab uprisings.

Since Salman took power in January, he has abandoned Riyadh's traditional reliance on Washington to do the heavy lifting in Middle East security, instead embarking on a war in Yemen and boosting support for rebels against Assad in Syria.

It showed both Riyadh's increasing sense of independence, and willingness to work with regional allies instead of Washington. But it also a more transactional approach to relations with the US.

Unlike at some earlier points in the relationship, cooperation now appears to be limited, either to instances where interests directly overlap, or to support in one area in exchange for another.

Obama has been keen for Washington's allies to take on a bigger role in regional security issues as part of his efforts to reduce American involvement in the Middle East's many messy conflicts.

On Wednesday, Jeff Prescott, National Security Council senior director for Iran, Iraq, Syria and the Gulf, told reporters that Washington had been "looking to support Saudi efforts to build their own capabilities and build their own capacity to act."

NUCLEAR FALLOUT

The clearest evidence of that shift in the relationship is in Yemen, where Riyadh formed a coalition of 10 Arab states in air strikes against the Iran-allied Houthi forces in March, a campaign that relies heavily on American military assistance.

While US officials have talked of a need for Riyadh to do more to avoid civilian casualties, Washington's role in accelerating weapons delivery as well as intelligence and logistical aid has been indispensable to the Saudi war effort.

Many Saudis regard such material American support in the Yemen war as part of Obama's efforts to reassure the kingdom and its Gulf allies that the Iran nuclear deal does not mean Washington will allow Tehran to dominate the Middle East.

Riyadh is convinced that Iran is bent on achieving regional hegemony through proxy militias in Lebanon, Syria, Iraq and Yemen, thereby destabilising the entire region and undermining the kingdom's own stability.

Washington, while critical of Iran's regional role, does not see it in such dire terms.

The Saudis fear that by relieving sanctions on Iran, the nuclear deal will give Tehran more money and political freedom to back clients like Hezbollah in Lebanon and Iraq's Shi'ite militias, as well as allies like Yemen's Houthis.

"Saudi Arabia is way more concerned about current regional matters, not the hypothetical threat from an Iranian bomb. Washington should not get free support for the deal unless they commit more to current Saudi concerns in the region," said Khashoggi of the Al-Arab channel.

Riyadh's public response to the deal, combining lukewarm praise with the proviso that its success would depend on tough inspections, was accompanied by private expressions of concern by Saudi officials.

SYRIAN CRISIS

The next big test of ties between the two countries is likely to be in Syria, where Saudi Arabia has frequently described Assad's use of air raids and artillery on civilian areas as genocidal, and has described Iran's support for militias there as constituting an occupation.

While Obama has said Assad can have no role in Syria's future, his bigger focus has been on the militant threat posed by Islamic State.

While Riyadh helped corral regional states to join US-led air strikes against IS in Syria last year, it has been disappointed by Washington's limited efforts to train and arm non-jihadist rebel groups.

Ben Rhodes, the US deputy national security adviser, alluded to Washington's own worries that by backing the Syrian opposition too fulsomely, there was a risk of inadvertently strengthening militant groups, in comments to media on Wednesday.

"We want to make sure that when we are working with our Gulf partners and Turkey ... that we have a common view in terms of which opposition deserves our support and that we're seeking to isolate more extremist elements of the opposition," he said.

Saudi Arabia, along with Qatar and Turkey, appears to have stepped up the backing of rebel groups this year, helping them to achieve advances against Assad. If its Yemen campaign succeeds, with American help, Riyadh may be emboldened to increase that support still further.  - Reuters