* Crown prince to return to Saudi Arabia
* Abdullah was admitted to hospital on Friday
* Kingdom seeking to prevent power vacuum
(Adds crown prince's return, quotes, background)
RIYADH, Nov 21 (Reuters) - Saudi Arabia's King Abdullah will leave for the United States on Monday for medical checks, while Crown Prince Sultan will return from a holiday abroad, the state news agency SPA said on Sunday.
Political stability in the monarchy is of global concern. The Gulf Arab state controls more than a fifth of the world's crude reserves, is a vital U.S. ally in the region, a major holder of dollar assets and home to the biggest Arab bourse.
Western diplomats said the king's departure and crown prince's sudden return indicate the absolute monarchy, which has no political parties or elected parliament, is seeking to prevent a power vacuum and reassure Washington and other allies. Abdullah, seen by Washington as a moderate at the helm of a pivotal Muslim country, was admitted to hospital on Friday after a blood clot complicated a slipped disc he suffered the week before.
"The king will leave on Monday for the United States to complete medical tests," the Saudi Press Agency SPA said.
Crown Prince Sultan, who has had unspecified health problems over the past two years, meanwhile would return to Saudi Arabia on Sunday evening from Morocco where he has been since August.
The king is thought to be 86 or 87 and Sultan is only a few years younger.
The United States is keen to see reforms continue after the Sept. 11 attacks of 2001 on U.S. cities brought Saudi Arabia's puritanical Wahhabi form of Sunni Islam to the top of global concerns. Fifteen of the 19 al Qaeda attackers were Saudi.
Saudi Arabia has become key to global efforts to fight al Qaeda. A Saudi intelligence tip-off helped Western governments stop package bombs destined for the United States that were sent on planes out of Yemen last month.
PRINCE NAYEF
Interior Minister Prince Nayef, comparatively youthful at around 76, was appointed second deputy prime minister in 2009 in a move which analysts say will secure leadership in the event of serious health problems afflicting the king and crown prince.
The position does not guarantee that Nayef would become king but places him in a strong position.
Analysts see jostling for position at the top of the ruling family.
Last week the king transferred control of the National Guard, an elite Bedouin corps that handles domestic security, to his son Mitab.
With both the king and crown prince indisposed, Prince Nayef has featured heavily in state media over the past week.
The veteran security chief was in an ebullient mood when he met reporters in Mecca before the haj pilgrimage last week and state media made a formal announcement that he would oversee the haj in the king's place, receiving guests there in recent days.
Nayef is seen as a hawk on a range of issues. Analysts say he appears lukewarm about the social and economic reforms the king has promoted, including attempts to reduce the influence of the hardline clerical establishment in a country that imposes strict Islamic sharia law.
Analysts say the ruling Al Saud family, which founded the kingdom with the help of Wahhabi clerics in 1932, needs to promote younger princes to dispel the image of gerontocracy.
So far only sons of state founder Abdul-Aziz Ibn Saud can become kings of which about 20 are left, some in ill health.
Prince Salman, in his 70s, will return to the country on Tuesday to resume duties as governor of Riyadh, SPA also said on Sunday. He underwent a spine surgery in the United States in August and remained outside the kingdom for recuperation.
Salman is a full brother of both Crown Prince Sultan and Prince Nayef. (Reporting by Ulf Laessing; Writing by Andrew Hammond; editing by Myra MacDonald)
Our Standards: The Thomson Reuters Trust Principles.
