King Abdullah of Saudi Arabia, the powerful U.S. ally who joined Washington's fight against Al Qaeda and sought to modernize the ultraconservative Muslim kingdom has died at 90, according to Saudi state TV.

His successor is his 79-year-old half-brother, Prince Salman, who recently has taken on the ailing Abdullah's responsibilities.

The announcement came in statement read by a presenter on Saudi state TV, which aired video of worshippers at the Kaaba in Mecca.

Saudi state TV said he died after midnight Friday.

A former American diplomat close to the Saudi royal family told Fox News the death of King Abdullah, coupled with the collapse of the government in Yemen, is a "worst case scenario" for the U.S. because current events are allowing Iran to extend its reach and influence in the region.

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With the collapse of President Hadi's government in Yemen, the former diplomat said Teheran's influence is now seen in at least four Middle Eastern capitals - Sana'a in Yemen, Baghdad in Iraq, Damascus in Syria, and to a lesser extent in Beirut, Lebanon.

In a written statement issued shortly after the announcement of Abdullah's death, President Obama expressed condolences and said, " I always valued King Abdullah's perspective and appreciated our genuine and warm friendship. As a leader, he was always candid and had the courage of his convictions.

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

Obama visited with the ailing king in his desert compound last March.

Secretary of State John Kerry, who was in London for a meeting of the coalition fighting Islamic State militants in Iraq and Syria, called Abdullah "a brave partner in fighting violent extremism who proved just as important as a proponent of peace."

Defense Secretary Chuck Hagel called the king "a powerful voice for tolerance, moderation and peace -- in the Islamic world and across the globe."

Former President George H.W. Bush said he was "deeply saddened to learn of the passing of my dear friend and partner King Abdullah. As President, I found His Majesty always to be a wise and reliable ally, helping our nations build on a strategic relationship and enduring friendship dating back to World War II."

Sen. John McCain, R-Ariz., called Abdullah "an important voice for reform in Saudi Arabia. He pushed for the modernization of the education system, curbed the authority of the religious police, and extended women the right to vote and run in municipal elections."

More than his guarded and hidebound predecessors, 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.

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. He 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.

Abdullah 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, Abdullah 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.

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

Abdullah became de facto ruler in 1995 when a stroke incapacitated Fahd. Abdullah was believed to have long rankled at the closeness of the alliance with the United States, and as regent he pressed Washington to withdraw the troops it had deployed in the kingdom since the 1990 Iraqi invasion of Kuwait. The U.S. finally did so in 2003.

When President George W. Bush came to office, Abdullah again showed his readiness to push against his U.S. allies.

In 2000, Abdullah convinced the Arab League to approve an unprecedented offer that all Arab states would agree to peace with Israel if it withdrew from lands it captured in 1967. The next year, he sent his ambassador in Washington to tell the Bush administration that it was too unquestioningly biased in favor of Israel and that the kingdom would from now on pursue its own interests apart from Washington's.

Bush soon after advocated for the first time the creation of a Palestinian state alongside Israel.

The next month, the Sept. 11, 2001 terror attacks took place in the United States, and Abdullah had to steer the alliance through the resulting criticism. The kingdom was home to 15 of the 19 hijackers, and many pointed out that the baseline ideology for Al Qaeda and other groups stemmed from Saudi Arabia's Wahhabi interpretation of Islam.

When Al Qaeda militants in 2003 began a wave of violence in the kingdom aimed at toppling the monarchy, Abdullah cracked down hard. For the next three years, security forces battled militants, finally forcing them to flee to neighboring Yemen.

There, they created a new Al Qaeda branch, and Saudi Arabia has played a behind-the-scenes role in fighting it.

The tougher line helped affirm Abdullah's commitment to fighting Al Qaeda. He paid two visits to Bush -- in 2002 and 2005 -- at his ranch in Crawford, Texas.

When Fahd died in 2005, Abdullah officially rose to the throne. He then began to more openly push his agenda.

His aim at home was to modernize 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.

Abdullah was a strong supporter of education, building universities at home and increasing scholarships abroad for Saudi students.

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 a 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.

One of his most ambitious projects was a Western-style university that bears his name, the King Abdullah University of Science and Technology, which opened in 2009. Men and women share classrooms and study together inside the campus, a major departure in a country where even small talk between the sexes in public can bring a warning from the morality police.

But he treaded 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.

Regionally, perhaps Abdullah's biggest priority was to confront Iran, the Shiite powerhouse across the Gulf.

Worried about Tehran's nuclear program, Abdullah told the United States in 2008 to consider military action to "cut off the head of the snake" and prevent Iran from producing a nuclear weapon, according to a leaked U.S. diplomatic memo.

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

The Associated Press contributed to this report.