King asked Crown Prince Sultan to fly home to run the kingdom during his absence
Saudi Arabia's King Abdullah flew to the United States for medical treatment on Monday, while Crown Prince Sultan hurriedly returned from abroad to govern the world's largest oil exporter.
The kingdom is keen to show its allies in Washington and elsewhere there will be no power vacuum as health problems beset its octogenarian rulers, but the question of whether a reformist or a conservative will take over remains a matter of concern.
Abdullah, thought to be around 86 or 87, asked Crown Prince Sultan to fly home from Morocco to run the kingdom during his absence.
The king will be seeking treatment after a blood clot complicated a slipped spinal disc, the state news agency SPA said. It did not say when Abdullah would be back.
With both Abdullah and Sultan in their 80s, speculation arose that conservative Interior Minister Prince Nayef, at a relatively youthful 76, could take over running the affairs of state some time in the near future.
Diplomats say Sultan, who is also defence minister and has major health problems of his own, has been much less active during his convalescence in Morocco.
Abdullah appointed his half-brother Nayef second deputy prime minister in 2009 in a move that analysts say will secure the leadership in the event of serious health problems afflicting the king and crown prince and improve Nayef's chances of one day being king.
Diplomats in Riyadh say Western governments concerned about the fate of social and economic reforms promoted by Abdullah have reservations about the ascent of Nayef, seen as a religious and social traditionalist.
Nayef long denied that the September 11 attacks of 2001 were carried out by Saudis or al Qaeda, suggesting they were carried out by supporters of Israel. He is seen as close to the powerful and hardline Saudi clerical establishment blamed by Washington for encouraging an ideology that promotes bigotry and fanaticism.
King Abdullah, who came to power in 2005, is the sixth leader of Saudi Arabia, whose political stability is of regional and global concern. It controls more than a fifth of the world's crude oil reserves, is a vital US ally in the region, a major holder of dollar assets and home to the biggest Arab bourse.
As home to Islam's holiest sites as well as birthplace of al Qaeda leader Osama bin Laden, Saudi Arabia is key to global efforts to fight Islamic militancy.
Washington wants Riyadh to continue social and economic reforms promoted by Abdullah that were seen as crucial after a group composed of mainly Saudis carried out the September 11 attacks of 2001.
But confusion still swirls over the real state of health of both Abdullah and Sultan and what will happen to Abdullah's policies.
Diplomats say there has been uncertainty about Abdullah's health since he cancelled a visit to France in July.
A series of official announcements over the past week on the king's health reflect a desire to reassure Washington that the ruling family's grasp of affairs remains firm in tense times.
The hasty return of Sultan from a three month break in Morocco was more low key than his return last year from treatment for unspecified health problems. Then the half-brother to Abdullah, only a few years his junior, was met with a Bedouin sword dance.
Saudi state television showed Sultan at Riyadh airport, where he was greeted by senior princes of the Saudi royal family, which may soon face the test of managing an orderly transfer of power.
Abdullah was not present at the event.
While official media seek to present family unity, tensions remain between the senior princes over who will run the country and over securing positions for their sons in the future political architecture of the absolute monarchy.
Rulers have so far all been sons of founder Abdul Aziz Ibn Saud. (Reuters)
We pray that King Abdulla gets well soon and returns to the Kingdom! He is very much required back for the continuing bright future of the Kingdom.
Insha Allah, God will give health to our King Abdullah. We pray for his health and for Saudi Arabia as symbol of unity for muslim world.
I humbly request all my friends, colleages and fellow Pakistanis all over the world to pray for the good health of HM King Abdullah and return to the Kingdom soon.