King Abdullah bin Abdulaziz, who cautiously reformed Saudi Arabia amidst the Arab Spring revolts and Islamic State militants' threats, died on Friday at age 90.

The New York Times reports The Royal Court announced via a television broadcast the king had died bu did not reveal the cause of death.

A report by the official Saudi Press Agency said the king had suffered a lung infection and was admitted to a Riyadh hospital on Dec. 31, 2014.

The king's death increases the region's uncertainty as Saudi Arabia struggles with Iran for regional dominance.

The royal family moved swiftly to ensure a seamless transition of power in a nation that is the world's largest exporter of oil, the religious center of the Islamic faith and a close ally of the U.S. Abdullah's brother, Crown Prince Salman, announced on Saudi television the king had died and that he had accepted the throne.

"As our countries worked together to confront many challenges, I always valued King Abdullah's perspective and appreciated our genuine and warm friendship," President Obama said in a statement issued by the White House. "As a leader, he was always candid and had the courage of his convictions."

Vice President Joe Biden will lead the American delegation to pay respects and offer condolences.

Time reported last April the state of affairs between Saudi Arabia and the U.S. showed signs of stress following a series of disagreements. In at least two telephone calls, Abdullah reprimanded President Obama for encouraging democracy in the Middle East, saying it was dangerous.

Nonetheless, Saudi Arabia and the U.S. have remained close allies. Abdullah contested al-Qaida's militant interpretations of the faith as justifying terrorist acts. He had hundreds of militants arrested -- some of which were beheaded. He also sent pilots to join in an American-led campaign against the Islamic State.

King Abdullah was one of the world's wealthiest and most powerful men. He controlled a fifth of the world's known petroleum reserves. As the custodian of Islam's holiest sites, Mecca and Medina, he was also one of the faith's most important figures.

Abdullah's ascension to the throne was in August 2005, following the death of his predecessor, King Fahd, a half-brother, who died of a stroke at age 82.

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