RIYADH (AFP) - Saudi King Abdullah flew on Monday to the United States to undergo treatment for a herniated disc as analysts said his absence was not expected to impact stability in the OPEC kingpin and US ally. Before leaving for a medical centre specialising in spinal ailments, Abdullah, 86, issued a decree tasking his half-brother, Crown Prince Sultan bin Abdul Aziz, with the administration of state affairs, the royal court said. Sultan, who is himself in poor health, had returned home on Sunday from Morocco, where had spent nearly 12 weeks at his Agadir palace. King since 2005, Abdullah had reduced his duties since June over health problems which were not officially disclosed until the palace released a statement on November 12 saying he had a herniated disc. A spokesman for Health Minister Abdullah al-Rabeeah said the king was suffering from a lumbar spine disc prolapse complicated with a haematoma pressing on lower nerve roots causing pain a painful but treatable ailment. Sultan, who has been defence minister for the past five decades, is 79 years old according to an official biography but approaching 85 according to a specialist close to the royal family. He has been receiving cancer treatment for the past two years, diplomats and analysts say. The health minister said on Sunday that the king was in a stable state and is well, and will undergo further medical examination and continued treatment... at a centre specialising in spinal problems in the United States. Any transfer of power in the absolute monarchy, founded in 1932, is keenly watched by oil markets as the OPEC kingpin has the worlds largest proven crude reserves with more than 260 billion barrels. It is the leading oil exporter, pumping about 8.2 million barrels per day. The Saudi riyal is fixed to the US dollar, the government has some 500 billion dollars in reserves and Saudi banks are flush with cash. Analysts say it is premature to speculate about an imminent handover of power because of the health of the king and the crown prince, and rule out instability because of Abdullahs absence. Prince Nayef, 76, Sultans full brother and considered second in line to the throne, has also had unspecified treatment in the past 12 months. He has been interior minister for 35 years. I think there is nothing to worry about, said Abdul Aziz al-Sager, chairman of the Gulf Research Centre. The system is in place. Christopher Boucek of the Carnegie Foundation agreed. There was rarely instability in Saudi Arabia. The family always comes together to figure out what should be done very quickly. There will be no vacuum of power. London-based analyst Abdel Wahab Badrakhan said a television appearance by Abdullah last week to reassure Saudis about his health shows that his condition is not serious. He also said the crown princes return has put an end to speculation about the durability of the regime. There has been no overt reaction in currency markets to the announcement of Abdullahs ailment, and oil prices also shrugged off the news to rise above 82 dollars on Monday on the strength of the euro. On the Saudi stock market, the Tadawul all-shares index dropped just 0.3 percent on Sunday and rose again 0.3 percent on Monday. Before leaving, the king signed decrees keeping in their posts for another four years the mufti the kingdoms top Muslim official as well as his chief of staff, his assistant head of royal protocol and Saudi Arabias US ambassador. Last Wednesday, the king named his son Prince Mitaeb bin Abdullah a member of the council of ministers and head of the national guard, a post he himself had held since 1962. Abdullah also accepted the resignation for health reasons of his brother Prince Badr bin Abdul Aziz as deputy head of the national guard, which protects strategic sites such as oil installations as well as the royal family. Saudi royals who have dominated the government for 30 years or more are all aged and have been treated for various health problems, usually never defined, during the past year.