RIYADH, Saudi Arabia » Saudi Arabia’s aged ruler will fly to the United States for medical tests over a blood clot, according to a Saudi official, in a development that would renew questions about succession in the oil-rich kingdom.

The 86-year-old King Abdullah 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 late Saturday. The official didn’t say which hospital would receive the king.

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

Crown Prince Sultan, 85, will arrive in Riyadh on Sunday to oversee the kingdom, the official added.

Prince Sultan, the defense minister, has spent over a year in the U.S. and Morocco for medical treatment of his own. He underwent surgery in New York in February last year before going to his palace in Agadir, Morocco, to recuperate.

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

Earlier this week, King 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 his deputy prime minister Prince Nayef bin Abdulaziz, the king’s half brother and powerful interior minister.

Abdullah assumed the throne in August 2005 after the death of his long-ailing half brother, King Fahd. He had already been a de-facto ruler for half a decade and the transition to the leadership of the key U.S. ally and oil giant passed smoothly.

The unstable health condition of the king and his crown prince raises questions about the complicated issue of royal succession in the oil-rich kingdom.

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