Saudi Arabia's King Abdullah has died, aged 90.

The death was announced on Saudi state TV by a presenter who said the king died at 1am local time.

His successor was announced as his 79-year-old half-brother, Prince Salman, according to a Royal Court statement carried on the Saudi Press Agency.

Prince Salman was King Abdullah's crown prince and had recently taken on some of the ailing king's responsibilities.

King Abdullah was a powerful US ally who joined Washington's fight against al Qaida and sought to modernise the ultraconservative Muslim kingdom with incremental but significant reforms, including nudging open greater opportunities for women.

More than his guarded and hidebound predecessors, King Abdullah assertively threw his oil-rich nation's weight behind trying to shape the Middle East.

His priority was to counter the influence of rival, mainly Shiite Iran wherever it tried to make advances. He and fellow Sunni Arab monarchs also staunchly opposed the Middle East's wave of pro-democracy uprisings, seeing them as a threat to stability and their own rule.

He backed Sunni Muslim factions against Tehran's allies in several countries, but in Lebanon for example, the policy failed to stop Iranian-backed Hezbollah from gaining the upper hand.

And Tehran and Riyadh's colliding ambitions stoked proxy conflicts around the region that enflamed Sunni-Shiite hatreds - most horrifically in Syria's civil war, where the two countries backed opposing sides.

Those conflicts in turn increased Sunni militancy that returned to threaten Saudi Arabia.

And while the king maintained the historically close alliance with Washington, there were frictions as he sought to put those relations on Saudi Arabia's terms.

King Abdullah was constantly frustrated by Washington's failure to broker a settlement to the Israel-Palestinian conflict.

He also pushed the Obama administration to take a tougher stand against Iran and to more strongly back the mainly Sunni rebels fighting to overthrow Syrian President Bashar Assad.

He was born in Riyadh in 1924, one of the dozens of sons of Saudi Arabia's founder, King Abdul-Aziz Al Saud. Like all Abdul-Aziz's sons, he had only rudimentary education.

Tall and heavyset, he felt more at home in the Nejd, the kingdom's desert heartland, riding stallions and hunting with falcons.

His strict upbringing was exemplified by three days he spent in prison as a young man as punishment by his father for failing to give his seat to a visitor, a violation of Bedouin hospitality.

He was selected as crown prince in 1982 on the day his half-brother Fahd ascended to the throne.

The decision was challenged by a full brother of King Fahd, Prince Sultan, who wanted the title for himself. But the family eventually closed ranks behind him to prevent splits.

He became de facto ruler in 1995 when a stroke incapacitated King Fahd, and rose to throne when King Fahd died in 2005.

King Abdullah had more than 30 children from around a dozen wives. 

Prince Muqrin, 69, a former head of intelligence in Saudi Arabia and half-brother to both Prince Salman and King Abdullah, was announced as the kingdom's crown prince.

President Barack Obama expressed condolences and offered sympathy to the people of Saudi Arabia upon the death of the king.

Mr Obama, who visited the ailing king in his desert compound last March, praised him for taking "bold steps" in advancing the Arab Peace Initiative.

The president credited the king for being dedicated to the education of his people and greater outreach to the international community, and said one of his legacies was the strength of the relationship between the US and Saudi Arabia.

He said he "valued King Abdullah's perspective and appreciated our genuine and warm friendship" and added that his advice was always candid. 

King Abdullah was believed to have long rankled at the closeness of the alliance with the US, and as regent he pressed Washington to withdraw the troops it had deployed in the kingdom since the 1990 Iraqi invasion of Kuwait. The US finally did so in 2003. 

His aim at home was to modernise the kingdom to face the future. One of the world's largest oil exporters, Saudi Arabia is fabulously wealthy, but there are deep disparities in wealth and a burgeoning youth population in need of jobs, housing and education.

More than half the current population of 20 million is under the age of 25. He was a strong supporter of education, building universities at home and increasing scholarships abroad for Saudi students.

King Abdullah for the first time gave women seats on the Shura Council, an unelected body that advises the king and government.

He promised women would be able to vote and run in 2015 elections for municipal councils, the only elections held in the country. He appointed the first female deputy minister in 2009. Two Saudi female athletes competed in the Olympics for the first time in 2012, and a small handful of women were granted licenses to work as lawyers during his rule.

But he moved carefully in the face of the ultraconservative Wahhabi clerics who hold near total sway over society and, in return, give the Al Saud family's rule religious legitimacy.

After the September 11, 2001 terror attacks took place in the United States, King Abdullah had to steer his country's alliance with Washington through the resulting criticism.

The kingdom was home to 15 of the 19 hijackers, and many pointed out that the baseline ideology for al Qaida and other groups stemmed from Saudi Arabia's Wahhabi interpretation of Islam.

When al Qaida militants in 2003 began a wave of violence in the kingdom aimed at toppling the monarchy, King Abdullah cracked down hard.

For the next three years, security forces fought militants, finally forcing them to flee to neighbouring Yemen. There, they created a new al Qaida branch, and Saudi Arabia has played a behind-the-scenes role in fighting it.