RIYADH, Saudi Arabia—The heir to the Saudi throne, Crown Prince Sultan bin Abdel Aziz Al Saud, died abroad on Saturday after an illness, state TV said. The death of the 85-year-old prince opens questions about the succession in the critical, oil-rich US ally.

Sultan was the half-brother of Saudi Arabia’s King Abdullah, who is two years older than him and has also been ailing and underwent back surgery last week.

The most likely candidate to replace Sultan as Abdullah’s successor is Prince Nayef, the powerful interior minister in charge of internal security forces. After Sultan fell ill, the king gave Nayef—also his half-brother—an implicit nod in 2009 by naming him second deputy prime minister, traditionally the post of the second in line to the throne.

The announcement did not say where outside the kingdom he died or elaborate on Sultan’s illness but Saudi official circles in Riyadh said he passed away at a hospital in New York. Saudi royal family members typically received medical treatment in New York.

Sultan, who was the kingdom’s deputy prime minister and the minister of defense and aviation, has had a string of health issues. He underwent surgery in New York in February 2009 for an undisclosed illness and spent nearly a year abroad recuperating in the United States and at a palace in Agadir, Morocco.

“With grief, King Abdullah bin Abdel Aziz mourns the death of Sultan bin Abdel Aziz Al Saud, crown prince and his brother,” the palace said. The statement, which was carried on the official Saudi Press Agency, added that Sultan’s funeral will be held on Tuesday afternoon in Riyadh at the Imam Turki bin Abdullah mosque.

For the first time, however, the mechanism of picking the next crown prince is not entirely clear. It is possible the king will for the first time put the decision of his heir to the Allegiance Council, a body Abdullah created a decade ago as one of his reforms, made up of his brothers and nephews with a mandate to determine the succession. That would open the choice up to a degree of debate with the top echelons of the royal family. Nayef, however, will still be the front-runner.

Traditionally, the king names his successor. But Abdullah formed the council in order to modernize the process and give a wider voice to the choice. When it was created, it was decided that the council would act when Sultan rose to the throne and his crown prince had to be named; however, it was not specified whether it would be used if Sultan died before the king. The choice of whether to evoke the council now will likely be made by Abdullah.

Saudi Arabia has been ruled since 1953 by the sons of its founder, King Abdul-Aziz, who had over 40 sons by multiple wives.

Anyone who rises to the throne is likely to maintain the kingdom’s close alliance with the United States. But it would have an internal impact. Abdullah has been seen as a reformer, making cautious changes to improve the position of women—such as granting them the right to vote in elections scheduled for 2015—and seeking to modernize the kingdom despite some backlash from the ultraconservative Wahhabi clerics who give the royal family the religious legitimacy needed to rule. Nayef, however, is often seen as closer to the clerics.

(AP)