JEDDAH, Saudi Arabia (Reuters) - Saudi King Abdullah, who underwent surgery last year for back-related problems, will undergo an operation on his back on Monday, medical sources said.
The monarch, who is in his late 80s, went into hospital late on Sunday for an operation to tighten the ligaments around his third vertebra, the state news agency SPA reported.
King Abdullah has ruled Saudi Arabia since 2005 and has introduced cautious reforms in the world's top oil exporter.
Saudi stability is of global concern. A key U.S. ally, the top OPEC producer holds more than a fifth of world oil reserves.
Unlike in European monarchies, the line of succession does not move directly from father to eldest son, but has moved down a line of brothers born to the kingdom's founder King Abdulaziz Ibn Saud, who died in 1953.
Abdullah's immediate heir, Crown Prince Sultan, who is also in his mid 80s, has been in the United States since June for medical treatment.
The next-in-line after Sultan is assumed to be Prince Nayef, the interior minister since 1975, who is in his late 70s and has a reputation as more conservative than his elder brothers.
Abdullah -- who had already ruled as de facto regent since King Fahd suffered a stroke in 1995 -- had two rounds of back surgery in the United States last year after suffering a herniated disc, leading to a three-month recuperation period outside the kingdom.
"I think the King... will be delegating his authority to Prince Nayef," said Khaled al-Dakhil, a professor of political science in Riyadh. "He is third in the ranks."
Nayef was made second-deputy prime minister in 2009 and has assumed day-to-day control of the kingdom before, during concurrent absences of his two elder brothers Abdullah and Sultan.
The leader of the West's foremost Gulf ally has given Saudi backing to U.S.-led efforts to confront and constrain Islamist militant groups, including al Qaeda, and has pushed Washington to support greater rights for Palestinians.
He has brought about reforms aimed at liberalising the economy, giving greater technical, rather than religious, emphasis to education, and allowing women more rights.
As for the energy sector, King Abdullah is seen to have supported a moderate oil policy, raising production to prevent price spikes during supply outages from other countries.
(Reporting By Asma Alsharif; Writing By Angus McDowall; Editing by Sami Aboudi and Maria Golovnina)
