JEDDAH, Saudi Arabia (Reuters) - Saudi Arabia's King Abdullah will undergo a back operation next week to tighten a loose ligament in his back, the Royal Court said in a statement carried by state news agency SPA on Sunday.

"The operation will take place, God willing, next week at King Abdulaziz Medical City for the National Guard in Riyadh," SPA quoted the Royal Court statement as saying.

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 statement did not say whether Abdullah would remain in charge of government of the world's top oil exporter during his treatment, but Crown Prince Salman acts as his deputy. Abdullah, who took power in 2005 after the death of King Fahd, named Salman, 13 years his junior, as his heir apparent in June.

Saudi stability is of global concern in the Sunni Muslim-dominated kingdom. The key U.S. ally holds more than a fifth of world petroleum reserves and is the birthplace of Islam.

Analysts said they expected Salman to continue the gradual social and economic reforms adopted by King Abdullah as well as Saudi Arabia's moderate oil pricing policy.

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 Shiites 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.

(Reporting by Asma Alsharif; Editing by Ralph Gowling and Jason Webb)