Saudi Arabia's King Abdullah bin Abdulaziz has died aged 90.

The OPEC-kingpin's royal court released a statement confirming the death and announcing the late monarch's 79-year-old half-brother Crown Prince Salman as his successor.

He reportedly died at 1am this morning local time and will be laid to rest in the capital city of Riyadh after the noon prayer today.

King Abdullah, who took power of the Middle Eastern nation in 2005 after the death of his half-brother King Fahd, has been in hospital since December with pneumonia.

King Abdullah was largely seen as the de facto ruler of Saudi Arabia since 1995, when King Fahd suffered a debilitating stroke.

King Abdullah's successor, Saudi Crown Prince Salman bin Abdulaziz Al Saud. (AAP) (AAP)

In recent years, his advanced age and poor health had raised concerns about the future leadership of one of the world's key oil producers.

After ascending the throne, the King had launched projects to build new economic cities, universities and high-speed railways. In 2011, he withstood the convulsions of the Arab Spring uprisings that ousted several leaders in the region.

More recently, Saudi Arabia has been among several Gulf countries taking part in a US-led air campaign against the Islamic State jihadist group in Syria.

King Abdullah's half-brother Salman, 79, was named crown prince in June 2012 following the death of Prince Nayef bin Abduzlaziz. Salman had been representing the king at most recent public events because of the monarch's poor health.

In March 2014, King Abdullah named his half-brother Prince Moqren as a second crown prince, in an unprecedented move aimed at smoothing succession hurdles. Moqren, who was born in 1945, is the youngest of Abdulaziz's sons.

US President Barack Obama, pictured here with King Abdullah, has expressed his sympathies at the leader's death. (AAP)

Since the death in 1952 of King Abdulaziz al-Saud, the founder of Saudi Arabia, the throne has systematically passed from one of his sons to another. But many of them are old or have died. The former crown princes Sultan and Nayef died in 2011 and 2012 respectively.

US President Barack Obama has expressed his personal sympathies on the passing of King Abdullah.