Saudi Arabia's King Abdullah was admitted to a hospital in Riyadh on Friday night to undergo back surgery to tighten a loose ligament, the state's SPA news agency said.
The king, in his late 80s, underwent an operation to tighten ligaments around his third vertebra in October of last year and had two rounds of back surgery in the United States in 2010 after suffering a herniated disc, leading to a three-month recuperation period outside the kingdom.
The stability of Saudi Arabia, the world's largest oil exporter and a key U.S. ally, is of global concern. The kingdom holds more than a fifth of world crude reserves and is the birthplace of Islam.
"The Custodian of the Two Holy Mosques, King Abdullah bin Abdulaziz al-Saud, may God keep him, arrived at the National Guard's King Abdulaziz Medical City in Riyadh, where a surgery will be performed ... to tighten a loose ligament in the upper back," SPA said.
His heir apparent, Crown Prince Salman bin Abdulaziz, normally acts as his deputy in his absence.
King Abdullah, who took power in 2005 after the death of King Fahd, named his brother, 13 years his junior, heir apparent in June after the death of Crown Prince Nayef bin Abdulaziz.
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.
While it faced some protests from minority Shi'ite Muslims in its Eastern Province, Saudi Arabia avoided the kind of unrest that toppled leaders across the Arab world last year after it introduced generous social spending packages and issued a religious edict banning public demonstrations.