The heir to the Saudi throne, Crown Prince Sultan bin Abdul-Aziz, has died of an undisclosed illness. He was in his eighties.

His death was announced by the Saudi media, which said he died Saturday “outside of the kingdom following an illness.” Western news reports say he died while undergoing medial treatment in New York.

Crown Prince Sultan was one of the kingdom's most powerful figures. He was also the half brother of King Abdullah.

In late 2010, he governed the kingdom while King Abdullah underwent surgery in New York. Both the monarch and the crown prince have suffered health problems in recent years.

In 2009, Prince Sultan received treatment in the U.S. for an undisclosed illness. Prior to the visit, he had been diagnosed with cancer.

U.S. Secretary of State Hillary Clinton reacted to news of his death saying he was a “strong leader” and a “good friend to the United States over many years.” She commented Saturday from Tajikistan, a stop of her tour of Central Asia.

The crown prince had served as minister of Defense and Aviation since 1963. The Saudi embassy in Washington said he was instrumental in establishing the kingdom's modern armed forces.

He had previously served as minister of Transportation and governor of Riyadh.

Saudi news reports say Prince Sultan's funeral will take place Tuesday in Riyadh.

Saudi officials have not announced a successor. However, the most likely candidate appears to be the interior minister, Prince Nayef. In 2009, he was named second deputy prime minister, a traditional post in the chain of succession.