Riyadh - Saudi Arabia's elderly King Abdullah died on Friday and was replaced by his half-brother Salman as the absolute ruler of the world's top oil exporter and the spiritual home of Islam.Global leaders paid tribute to the late monarch, seen as a cautious reformer who led his kingdom through a turbulent decade in a region shaken by the Arab Spring uprisings and Islamic extremism.The royal court said in a statement that Abdullah, believed to be around 90, died at 01:00 local time, expressing its "great sadness and mourning".Salman, aged 79, had been defence minister and previously governor of the capital Riyadh.Another of the late monarch's half-brothers, Moqren, was named the new crown prince.Abdullah will be buried later on Friday following afternoon prayers, the statement said.Citizens will then be invited to pledge allegiance to the new monarch and the crown prince at the royal palace.The royal court did not disclose the cause of Abdullah's death, but he was hospitalised in December suffering from pneumonia and had been breathing with the aid of a tube.Under Abdullah, who took the throne in 2005, Saudi Arabia has been a key ally of Washington in the Arab world, most recently joining the US-led coalition carrying out air strikes against ISIS in Syria and Iraq.US President Barack Obama was quick to pay tribute to Abdullah as a valued ally."As our countries worked together to confront many challenges, I always valued King Abdullah's perspective and appreciated our genuine and warm friendship," Obama said in a written statement shortly after the monarch's death."The closeness and strength of the partnership between our two countries is part of King Abdullah's legacy."Other tributes came in from Japan, India and France, whose President Francois Hollande hailed Abdullah as "a statesman whose work profoundly marked the history of his country".As the top producer in the Organisation of the Petroleum Exporting Countries, Saudi Arabia has been the driving force behind the cartel's refusal to slash output to support oil prices, which have fallen by more than 50% since June.Oil prices surged Friday following Abdullah's death, amid uncertainty over whether the new king would maintain that policy.Royal family stalwartThe kingdom is also home to Islam's holiest sites, Mecca and Medina, and its role as a spiritual leader for Sunni Muslims has seen it vying for influence with Shiite-dominated Iran.Behind his thick, always jet-black moustache and goatee, Abdullah had a shrewd grasp of regional politics.Wary of the rising influence of Islamist movements, Saudi Arabia has been a generous supporter of Egyptian leader Abdel Fattah al-Sisi since the army's ouster of Mohamed Morsi of the Muslim Brotherhood.It has also played a key role in supporting opposition to Iran-backed President Bashar Assad of Syria, allowing US troops to use its territory to train rebel fighters.Salman, the new king, is widely expected to follow closely in Abdullah's footsteps, in foreign and energy policy as well as in making moderate reforms to the deeply conservative kingdom.Abdullah pushed through cautious changes while in power, challenging conservatives with moves such as including women in the Shura Council, an advisory body.He promoted the kingdom's economic development and oversaw its accession to the World Trade Organisation, tapping into the country's massive oil wealth to build new economic cities, universities and high-speed railways.But Saudi Arabia is still strongly criticised for a dismal human rights record, including the imprisonment of dissidents. It is also the only country in the world that does not allow women to drive.Salman is a stalwart of the royal family credited with transforming Riyadh during his half-century as governor from a backwater to a thriving capital.Recent years have seen concerns over his health after operations on his back, but Salman took on an increasingly high-profile role as Abdullah's own health issues forced him from the limelight.Challenges aheadAbdullah named Moqren as deputy crown prince last March, in an unprecedented move aimed at smoothing succession hurdles.Moqren, a former intelligence chief, was a trusted confidant of Abdullah with a reputation as a liberal.A former air force officer born in 1945, Moqren is the youngest son of King Abdul Aziz bin Saud, the founder of Saudi Arabia.Since King Abdul Aziz's death in 1952 the throne has systematically passed from one of his sons to another.Abdul Aziz had 45 recorded sons and Abdullah, Salman and Moqren were all born to different mothers.The new king will face some major challenges, especially as falling oil prices cut into state revenues.Saudi Arabia has managed to avoid the social upheaval that has shaken many of its neighbours in recent years, thanks in large part to massive public spending.The country has amassed enormous financial reserves, but has already projected a huge deficit of $38.6bn for this year.