Saudi Arabia's 86-year-old king flew Monday to the U.S. for medical treatment and left control of the world's top oil producer and key American ally in the hands of an 85-year-old half brother who has suffered his own serious health problems.

The smooth transfer of power from one brother to another served as a reminder of the advancing age of the generation of the royal Al Saud family that has ruled the kingdom for the past 60 years. It also revived a long-standing question that may be taking on greater urgency: Can the rulers maintain stability when it comes time to pass the throne to a new generation.

Before King Abdullah headed for the United States, Saudi officials had been making a strong push to reassure the public and its international allies that there is nothing to worry about.

In a news conference shown on state TV, Health Minister Abdullah al-Rabeeah said the king suffered a slipped disc. He said a blood clot was pressing on nerves in his back, causing him pain and so he was heading abroad for treatment.

"But I assure everyone that the king is in stable condition and enjoys good health and God willing will return in good health to lead this great nation," al-Rabeeah said.

In a country where personal issues within the royal family are often kept under strict wraps, authorities have sought to show they are being transparent about Abdullah's condition, aiming to dispel any speculation.

Pictures in newspapers over recent days have shown the king being pushed in a wheelchair — though still looking fairly hardy. On Monday, state media showed photos of Abdullah bidding farewell to officials at the airport. He was shown seated in a plush chair on the tarmac, an IV catheter sticking out of the back of his hand, as he kissed the cheeks of his second deputy prime minister Prince Nayef bin Abdulaziz.

It was not immediately known where Abdullah would be receiving treatment in the United States. A statement from the palace said only that he would be undergoing "medical tests."

In Washington, the State Department said it helped facilitate clearances for the king to come to the U.S. for treatment.

"The king is a valued partner," department spokesman P.J. Crowley said. "We wish him a speedy recovery. I don't know that we have any particular concerns about his health. We want to see him up and about as quickly as possible."

Before he left, Abdullah issued a royal decree mandating Crown Prince Sultan, his half brother and heir to the throne, to "administer the nation's affairs" in his absence.

Abdullah has temporarily handed over authorities in the past when he has travelled abroad for conferences or personal trips, though this was the first time for health reasons.

The 85-year-old Sultan — also the defense and aviation minister — has his own health issues: He underwent surgery in New York in February 2009 for an undisclosed illness and spent nearly a year abroad recuperating in the United States and at a palace in Agadir, Morocco.

Even since his return to Saudi Arabia in December last year, he has spent extended periods in Morocco — from which he had to be called home quickly on Sunday to be on hand to fill in during the king's absence. His illness has never been confirmed, but diplomats have said he has been treated for cancer.

The most likely candidate for the throne after Abdullah and Sultan is Prince Nayef, the powerful interior minister in charge of internal security forces. After Sultan fell ill, the king gave Nayef an implicit nod in 2009 by naming him second deputy prime minister, traditionally the post of the third in line.

Anyone who rises to the throne is likely to maintain the kingdom's close alliance with the United States. But there could be internal differences. Abdullah has been seen as a reformer, making incremental changes to improve the position of women, for example, and to modernize the kingdom despite some backlash from the ultra-conservative Wahhabi clerics who give the royal family the religious legitimacy needed to rule. Nayef, for example, is often seen as closer to the clerics.

Still, Nayef's place in line is not certain.

Modern Saudi Arabia was founded by King Abdul-Aziz in the 1930s, and after his death in 1953, the kingdom has been ruled by his sons — Abdul-Aziz had dozens of children from several wives. So far, five of his sons have ruled, each handing the throne to a brother or half brother with relative calm.

The succession has generally been by seniority among the sons — and when there have been disputes, the family has always quickly closed ranks to resolve them and ensure the stability on which their family's rule depends.

Abdullah rose to the throne in 2005 after the death of King Fahd, though he had already been a de-facto ruler for half a decade.

In an attempt to formalize the succession system, Abdullah in 2006 set up the Allegiance Council, a body that is composed of Abdul-Aziz's sons and grandsons, who will vote by a secret ballot to choose future kings and crown princes.

The council's mandate will not start until after the reigns of Abdullah and Sultan are over. That means it could weigh in on whether Nayef can step in.

The bigger question is what happens when the generation of Abdul-Aziz's sons runs out. The youngest of the seven or eight sons often cited as having the stature and experience to rule are in their mid-60s — suggesting the generation still has some time left in power.

But sooner or later, the throne will have to move on to the next generation, raising the potentially deeply divisive question of which Abdul-Aziz son will pass power to his own son. Abdullah's creation of the council aimed in part to provide a system for that transfer.

Each brother has sought to set up his sons in positions of power, in part to guarantee their line's position. Nayef's son Mohammed, for example, is chief of the powerful counterterrorism forces that took the lead in the fight against al-Qaida the past decade — making Mohammed the target of a suicide bomber who nearly killed him last year.

Earlier this month, Abdullah stepped down as head of the country's 260,000-strong elite National Guard and gave the post to his own son, Prince Mitab. On Monday before traveling, the king swore his son into his post, surrounded by seven of his others sons, who include a provincial governor and a royal adviser.


Associated Press writers Abdullah al-Shihri in Riyadh, Saudi Arabia, and Matthew Lee in Washington contributed to this report.