RIYADH: Saudi Arabia's King Abdullah has undergone successful back surgery at a hospital in the capital, Riyadh, to tighten a loose ligament, the royal court said in a statement carried by state media on Sunday.

The stability of Saudi Arabia, the world's largest oil exporter and a key US ally, is of global concern. The kingdom holds more than a fifth of world crude reserves and is the birthplace of Islam.

“A surgery was performed on the Custodian of the Two Holy Mosques, King Abdullah bin Abdulaziz, at the National Guard's King Abdulaziz Medical City in Riyadh on Saturday ... where a loose ligament in the upper back was tightened,” the statement, carried on state television and the SPA news agency, said.

“With God's help, the surgery ended at 0315 on Sunday morning ... and thanks be to God it was successful,” it added in Arabic.

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.

His heir apparent and brother, Crown Prince Salman, normally acts as his deputy in his absence.

King Abdullah, who took power in 2005 after the death of King Fahd, named Salman heir apparent in June after the death of Crown Prince Nayef bin Abdulaziz. Prince Salman is 13 years younger than Abdullah.

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 Shia 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.