Abdullah's poor health has once again ushered in questions around who will succeed him [EPA] 

Saudi Arabia's ruler will fly to the United States for medical tests over a blood clot, according to a Saudi official, in a development that renews questions about succession in the kingdom.

King Abdullah bin Abdulaziz, 86, is set to leave on Monday, three days after he was admitted to the hospital suffering from back pain due to a blood clot, the official said on Saturday.

The palace said on Friday that the king had a blood clot, which had created pressure on the nerves surrounding a ruptured disc.

Sultan bin Abdulaziz, the crown prince, will arrive in Riyadh on Sunday to oversee the kingdom, the official added.

Sultan, 85, who is also the defence minister, has spent over a year in the US and Morocco for medical treatment of his own.

He underwent surgery in New York in February last year before going to Morocco to recuperate.

The official spoke on condition of anonymity because he was not authorised to speak to the press.

Earlier this week, Abdullah stepped down as head of the country's National Guard and transferred the influential position to his son.

He also handed over his traditional supervisory duties for the annual Hajj pilgrimage to Nayef bin Abdulaziz, Abdullah's half-brother who is the deputy prime minister and interior minister.

Abdullah assumed the throne in August 2005 after the death of his long-ailing half brother, Fahd, the former king.

Abdullah had already been a de-facto ruler for half a decade and the transition to the leadership of Saudi Arabia passed smoothly.

Saudi Arabia, the world's biggest oil producer, is a close ally of the US.

In 2006, Abdullah set up the Allegiance Council, a body that is composed of the sons and grandsons of Saudi Arabia's founder, Abdulaziz, to 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.

It is not clear, however, what would happen if Sultan were to die before the end of Abdullah's reign, leaving a question as to whether the council would vote for a new crown prince or whether Nayef would automatically fill that position.

Source: Agencies