King Abdullah of Saudi Arabia, who came to the throne in old age and earned a reputation as a cautious reformer even as the Arab Spring revolts toppled heads of state and Islamic State militants threatened the Muslim establishment that he represented, died on Friday, according to a statement on state television. He was 90.

The Royal Court said in a statement broadcast across the kingdom that the king had died early Friday. The royal court did not disclose the exact cause of death. An announcement quoted by the official Saudi Press Agency said the king had a lung infection when he was admitted on Dec. 31 to a Riyadh hospital.

The king’s death adds yet another element of uncertainty in a region already overwhelmed by crises and as Saudi Arabia is itself in a struggle with Iran for regional dominance.

The royal family moved quickly to assure a smooth transition of power in a nation that is a close ally of the United States, the world’s largest exporter of oil and the religious center of the Islamic faith. In a televised statement, a brother of Abdullah, Crown Prince Salman, announced that the king had died and that he had assumed the throne.