RIYADH, Saudi Arabia — The heir to the Saudi throne, Crown Prince Sultan bin Abdul-Aziz Al Saud, died undergoing treatment for illness in New York. The death of the prince, who was in his 80s, opens questions about the succession in the critical, oil-rich U.S. ally.

Sultan was the younger half-brother of Saudi Arabia’s King Abdullah, who is 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, who is said to be closer to Islamic conservatives than the king. The king gave Nayef — also his half-brother — the implicit nod in 2009 by naming him second deputy prime minister, traditionally the post of the second in line to the throne.

State TV announced that Sultan died abroad, without specifying where. Saudi official circles in Riyadh said he died at a hospital in New York. According to a leaked U.S. diplomatic cable from January 2010, Sultan had been receiving treatment for colon cancer since 2009.