Saudi Arabia's King Abdullah bin Abdulaziz has died in hospital after almost a decade in power and his half-brother Crown Prince Salman has succeeded to the throne.

A statement from Saudi Arabia's royal court confirmed the monarch - believed to be aged 90 or 91 - passed away at 1.00am local time.

Abdullah, who took power in 2005 following the death of King Fahd, is expected to be buried later today following afternoon prayers.

"With great sadness and mourning His Royal Highness Crown Prince Salman bin Abdulaziz al-Saud, and all members of the family and the nation, express condolences for the Custodian of the Two Holy Mosques King Abdullah bin Abdulaziz al-Saud," the statement said.

The late monarch's half-brother Moqren has been confirmed as the new Saudi Crown Prince.

The public will be invited to pledge allegiance to the new monarch and Crown Prince at the royal palace later today.

Abdullah had run the country as de factor leader since the mid-1990s after his predecessor suffered a debilitating stroke.

He was admitted to hospital on 31 December suffering pneumonia and the royal court announced that he was breathing with the aid of a tube.

Sky's Middle East correspondent Sherine Tadros said the death comes after weeks of speculation and rumour about the monarch's health.

"He's been ill for several weeks. He was admitted to hospital in December with a liver infection and there have been rumours over the past few weeks that he had died," she said.

His successor King Salman, 79, became Crown Prince in June 2012 after the death of Prince Nayef bin Abdulaziz.

He has represented Abdullah at recent public events due to the late king's ailing health.

World leaders have paid tribute to Abdullah, who encouraged reforms to the oil-rich kingdom including greater freedom for women and economic deregulation.

During his reign, Abdullah worked to counter the influence of Saudi Arabia's chief rival Iran.

Abdullah and his fellow Sunni Arab monarchs also staunchly opposed the wave of pro-democracy uprisings across the Middle East since 2011.

Saudi Arabia has been among several Gulf countries taking part in a US-led air campaign against Islamic State.

His nation also unflinchingly led Gulf countries in ignoring calls by other producers to cut oil output to curb the sliding price of crude.

He also launched projects to build new economic cities, universities and high-speed railways.

US President Barack Obama expressed his condolences in the wake of the death and hailed Abdullah's "steadfast and passionate belief" in Saudi-American relations.

"As a leader, he was always candid and had the courage of his convictions," Mr Obama said in a statement.

"One of those convictions was his steadfast and passionate belief in the importance of the US-Saudi relationship as a force for stability and security in the Middle East and beyond.

"The closeness and strength of the partnership between our two countries is part of King Abdullah's legacy."

(Sky News)