RIYADH, Saudi Arabia (AFP) - Saudi King Abdullah, 86, will undergo more surgery on his back in the United States on Friday, the royal court said, nine days after his first operation to fix a herniated disc. King Abdullah will return for surgery to repair several vertebrae in his spine in a follow-up to his previous treatment... in accordance with the plan set by his medical team, the court said in a statement carried by the official SPA news agency. Abdullah flew to New York City on November 22 and was operated on two days later at New York Presbyterian Hospital for a debilitating herniated disc complicated by a haematoma that put pressure on his spine. The court had said the first operation, on a condition that had visibly slowed Abdullah for several months, was a success, and that the king walked on his own inside the hospital shortly afterwards. But the combination of his age and the rare announcement that he needed to be hospitalised in the US sparked widespread concern about the future of the leadership of the worlds most important source of oil and a key player in Middle East politics. Any hint at possible change in the absolute monarchy, founded in 1932, is keenly watched as the OPEC kingpin pumps about 8.2 million barrels of crude per day. Abdullah is credited with advancing much-needed reforms in the ultra-conservative Islamist state since he became king in 2005, and maintaining the country as a staunch ally of the US. But the other leading princes of the government, including Crown Prince Sultan bin Abdul Aziz, are mostly in their 70s and 80s and have held their jobs for decades. Many are believed to suffer chronic ailments that have not been reported publicly. Prince Sultan, believed to be about 84, has spent much of the past two years outside the country receiving treatment for what is believed to be cancer and convalescing in his palace in Agadir, Morocco. He returned from Morocco on November 21 to assume responsibility for the royal government in his half-brother Abdullahs absence, but has also not been very active. He did preside over the weekly cabinet meeting on Monday. A full brother of Sultan interior minister and second deputy prime minister Prince Nayef bin Abdul Aziz, 76, is believed to be assuming more operational control of the government for the meantime. Nayef, the countrys internal security czar for 35 years, is widely considered second in line to the throne.