The heir to the Saudi throne, Crown Prince Sultan bin Abdul-Aziz, has died of an undisclosed illness. He was in his mid-80's.

The death of King Abdullah's half-brother was announced by the Saudi media, which said he died Saturday in New York City where he had gone for treatment of recurring medical problems.

The Saudi monarchy announced King Abdullah was mourning the death of his brother, who was one of the kingdom's most powerful figures, serving as both defense minister and deputy prime minister. 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 expressed “great regret” over the death, calling Crown Prince Sultan “a valued friend of the United States” who was a strong supporter of the “deep and enduring partnership” between the two countries forged almost 70 years ago.

Saudi officials said the funeral will take place Tuesday in Riyadh.

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

Saudi officials have not announced a successor, but 86-year-old King Abdullah has indicated his preferences, naming Interior Minister Prince Nayaf as second deputy prime minister in 2009, a traditional post in the chain of succession.

Prince Nayaf 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 the current king. Crown Prince Sultan's son Khaled is expected to take over his father's duties at the defense ministry. Another son, Prince Bandar, served as ambassador to the U.S. for more than 20 years.

King Abdullah, Prince Nayaf and the late crown prince are all the sons of the founder of the modern Arab nation, Abdul-Aziz, known as Ibn Saud.