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 medical treatment in New York.

Crown Prince Sultan was one of the kingdom's most powerful figures. He was the half brother of King Abdullah and had served as the country's defense minister since 1963.

The crown prince was also the planned successor to the king. His death throws into question the future leadership of a country facing simmering unrest at home and challenges from regional rival Iran.

U.S. President Barack Obama has expressed “great regret” over the death of the Prince Sultan and called him “a valued friend of the United States.”

In late 2010, Prince Sultan 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.

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

Saudi officials have not announced a successor, but 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.

Prince Nayef is said to have closer ties to Saudi Arabia's powerful, puritanical clerics than the king, who has instituted reforms, including allowing women to take part in future elections.

The succession decision will be made by a special council, in a change of policy that was made by King Abdullah.

Political observers say whoever is chosen will likely continue the close alliance between the oil-rich nation and the United States,