* Saudi worried U.S. losing interest in the Middle East
* Help to Syria's anti-Assad rebels to be discussed
* Riyadh concerned about Iranian influence in the Gulf
By Jeff Mason
RIYADH, March 28 (Reuters) - U.S. President Barack Obama arrived in Riyadh on Friday, aiming to persuade King Abdullah Saudi Arabian concerns that America is slowly disengaging from the Middle East are unfounded.
Obama, on his first visit to the kingdom since 2009, was due to meet Abdullah and other senior princes of the ruling al-Saud family in the monarch's desert farm at Rawdat Khuraim northeast of the capital Riyadh.
Obama descended to a red carpet, where he was welcomed by a group of Saudi princes. Secretary of State John Kerry and National Security Adviser Susan Rice followed Obama down the stairs and into the receiving line.
While Saudi Arabia, the world's largest oil exporter, supplies less petroleum to the United States than in the past, safeguarding its energy output remains important to Washington, as does its cooperation in combating al Qaeda.
Saudi rulers are hoping for the United States to shift its position on support for Syrian rebels, whom Riyadh has backed in their battle to oust President Bashar al-Assad.
They have previously fretted about Washington's reluctance to allow the supply of surface-to-air missiles, sometimes known as manpads, for fear they could end up in the hands of militants outside of Syria.
Deputy National Security Adviser Ben Rhodes said coordination with the kingdom on Syria policy, particularly regarding providing assistance to the Syrian rebels, had improved.
"That's part of the reason why I think our relationship with the Saudis is in a stronger place today than it was in the fall when we had some tactical differences about our Syria policy," he told reporters on Air Force One.
But he added Washington still had concerns over the supply of manpads to rebels, and that one of the main topics Obama and Abdullah would discuss would be how to empower the moderate opposition to counter Assad and isolate extremist groups.
King Abdullah and his family believe it is a strategic imperative to end Assad's rule to block what they see as a threat of Iranian domination in Arab countries, a view not shared by Washington.
The Saudis hope that by strengthening the rebels, they can change the balance of power on the battlefield enough to make Assad's main foreign backers more open to the idea of a political transition that involves a change of government.
However, Obama has shown himself wary of being drawn into another conflict in the Muslim world after working hard to end or reduce American military involvement in Iraq and Afghanistan.
The Saudis also want more reassurance on American intentions regarding talks over Iran's nuclear programme, which might eventually lead to a deal that ends sanctions on Tehran in exchange for concessions on its atomic facilities.
Riyadh fears such a deal could come at the expense of Sunni Arabs in the Middle East, some of whom fear that Shi'ite Iran will take advantage of any reduction in international pressure to spread its influence by supporting coreligionists.
An editorial in the semi-official al-Riyadh newspaper on Friday said that Obama did not know Iran as well as the Saudis, and could not "convince us that Iran will be peaceful".
"Our security comes first and no one can argue with us about it," it concluded.
Rhodes said Washington would not ignore Saudi concerns about Iranian action in the Middle East while it pursued a deal on Tehran's nuclear programme.
"We'll be making clear that even as we are pursuing the nuclear agreement with the Iranians, our concern about other Iranian behaviour in the region, its support for Assad, its support Hezbollah, its destabilising actions in Yemen and the Gulf, that those concerns remain constant," he said. (Additional reporting by Mark Felsenthal in Washington, Sami Aboudi in Dubai and Angus McDowall in Riyadh; Editing by Jon Boyle)