King Abdullah was admitted to a hospital three days ago for a back pain [EPA]

Saudi Arabia's ageing monarch has arrived in the United States to undergo medical treatment after a blood clot complicated a slipped spinal disc, the state department has said.

"Saudi King Abdullah bin Abdulaziz is currently visiting the US for medical treatment, and we hope for his speedy recovery," it said in a message posted on Twitter.

Before leaving Saudi Arabia on Monday, King Abdullah gave temporary authority to "administer the nation's affairs" to Sultan bin Abdulaziz, the crown prince. 

King Abdullah, 86, asked Sultan to fly home from Morocco, where he was on a three month holiday, to run the kingdom during his absence.

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

The king had been admitted to a Saudi hospital on Friday.

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

In 2006, he set up the Allegiance Council, a body comprising close relatives, 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 King Abdullah and Sultan are over.

With both King Abdullah and Sultan in their 80s, there has been speculation that Prince Nayef, the interior minister, could take over running the affairs of state some time in the near future.    

King Abdullah appointed his half-brother Nayef,76, 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.   

Source: Agencies