* Saudis 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 (Adds White House statement, dinner cancellation, new American ambassador)
By Jeff Mason
RIYADH, March 28 (Reuters) - U.S. President Barack Obama underlined Washington's "strong" relationship with Saudi Arabia in talks with King Abdullah on Friday, the White House said, on a visit intended to allay the kingdom's concerns that the decades-old alliance is weakening.
The elderly king, accompanied by a number of senior princes, had what appeared to be an oxygen tube connected to his nose at the start of the two-hour meeting at his desert farm at Rawdat Khuraim northeast of the capital Riyadh, witnesses said.
Saudi state television showed Obama, accompanied by U.S. Secretary of State John Kerry and National Security Adviser Susan Rice, listening attentively while King Abdullah spoke, gesticulating with both hands as he made a point.
A White House press statement after the meeting said the two countries were working together to address issues including the Syrian crisis, preventing Iran from developing nuclear weapons, combating extremism and supporting Middle East peace talks.
"In his meetings with King Abdullah in Riyadh, President Obama reiterated the significance the United States places on its strong relationship with Saudi Arabia, which has endured for over 80 years," the statement said.
The statement did not say if the two discussed the issue of human rights in Saudi Arabia, which an administration official had said on Thursday was expected to be part of the talks.
In the runup to the visit, officials had said Obama would aim to persuade the monarch that Saudi concerns that Washington was slowly disengaging from the Middle East and no longer listening to its old ally were unfounded.
AID FOR REBELS
Last year senior Saudi officials warned of a "major shift" away from Washington after bitter disagreements about its response to the "Arab Spring" uprisings, and policy towards Iran and Syria, where Riyadh wants more American support for rebels.
Overwhelmingly Sunni Muslim Saudi Arabia is backing the insurgents in their battle to oust Syrian President Bashar al-Assad, who is supported by Riyadh's rival, Shi'ite power Iran.
U.S. 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.
Rhodes added 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.
One area where Riyadh has long differed from Washington is in Obama's reluctance to supply rebels with surface-to-air missiles, sometimes known as MANPADS.
The Washington Post reported on Friday that the U.S. was ready to increase covert aid to Syrian rebels under a new plan which included training efforts by the CIA, and was considering supplying MANPADS.
The White House has not closed the door to the possibility of such a move in the future, but an official said its position had not changed.
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.
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.
FEARS OVER IRAN
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 co-religionists.
An editorial in the semi-official al-Riyadh newspaper on Friday said 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 for Hezbollah, its destabilising actions in Yemen and the Gulf, that those concerns remain constant," he said.
The Saudi king was accompanied in the talks by Crown Prince Salman, Prince Muqrin, who was named second-in-line to rule on Thursday, and Foreign Minister Prince Saud al-Faisal.
Powerful Interior Minister Prince Mohammed bin Nayef, who recently met top U.S. officials in Washington to discuss Syria, was not present according to a list of participants supplied by U.S. officials.
Also present was the new American ambassador in Riyadh, Joseph Westphal, whose appointment was confirmed by the Senate late on Wednesday, apparently in order to let him attend Friday's meeting. (Additional reporting by Mark Felsenthal in Washington, Sami Aboudi in Dubai and Angus McDowall in Riyadh; Editing by William Maclean and Andrew Roche)