The death of the octogenarian heir to the Saudi throne was expected Saturday to position a committed conservative who is a reliable, if choleric, ally of the United States as the ruler-in-waiting within the dynastic Saud family.

Crown Prince Sultan bin Abdul Aziz died in New York late Friday, according to the Saudi Press Agency, which did not specify a cause of death. Leaked U.S. diplomatic cables from 2010 said that Sultan, who was at least 80 and perhaps as old as 85, was being treated for colon cancer.

King Abdullah bin Abdul Aziz, possibly in consultation with the Allegiance Council, a 33-member group of senior figures from the Saud family, will choose his successor. And both family tradition and recent political developments suggest that the leading candidate is Prince Nayef bin Abdul Aziz, the interior minister who has been acting as de facto crown prince during Sultan’s prolonged illness and absences overseas for treatment.

A state funeral is planned for Tuesday in Riyadh for Sultan, the longtime defense minister who was in charge at the time of the first Gulf War. A possible successor to his Defense Ministry post is Sultan’s deputy and son, Prince Khaled.

“As Minister of Defense and Aviation for almost 50 years, Crown Prince Sultan dedicated himself to the welfare and security of his people and country and was a valued friend of the United States,” President Obama said in a statement.

Secretary of State Hillary Rodham Clinton, who was traveling in Central Asia when the news broke, praised the crown prince as a “good friend to the United States over many years, as well as a tireless champion for his country.”

Clinton declined to speculate about who might be named to replace him. “I do not think it’s appropriate to comment on what will be an internal decision by the kingdom of Saudi Arabia,” she said, “except to conclude by saying that our relationship with Saudi Arabia is strong and enduring.”

Middle East experts said they believe that Nayef’s ascension to the formal title of crown prince appears inevitable and is unlikely to prompt any major changes in the kingdom.

“Sultan has been sick for a long time, and I think the royal family has all but designated Nayef as his successor,” said Jon B. Alterman, the director of the Middle East Program at the Center for Strategic and International Studies. “People have assumed that Nayef was effectively the crown prince for a number of years, so there is no immediate change.”

As interior minister, Nayef, 78, has led an aggressive campaign against al-Qaeda and its allies, largely bottling up a movement that staged two major terrorist attacks in the kingdom in 2003. It was those attacks on residential compounds in the capital, Riyadh, rather than the Sept. 11, 2001, attacks on the United States, that prompted the crackdown.

“Nayef got religion, not after 9/11, but after the two attacks in 2003,” said Alterman. Earlier, Nayef had bristled at questions about the 15 Saudis among the 19 hijackers who struck New York and Washington and about the role of private Saudi money in financing al-Qaeda. And he infuriated some in the United States when he said in a 2002 interview with a Kuwait newspaper: “We still ask ourselves: Who has benefited from Sept. 11 attacks?” adding, in a reference to Jews, “I think they were the protagonists of such attacks.”

U.S. officials have said that counterterrorism cooperation and intelligence-sharing with Saudi Arabia have improved significantly in recent years. They have also described Nayef’s son, his deputy at the Interior Ministry, as a critical intermediary between Saudi authorities and Western intelligence agencies.

The son, Prince Mohammed bin Nayef, was the subject of a 2009 assassination attempt by al-Qaeda’s affiliate in Yemen in which a suicide bomber, pretending to turn himself in, blew himself up. The explosive device in that case was made by the same Saudi bomb-maker who fitted the so-called underwear bomber, the Ni­ger­ian who tried to bring down a commercial aircraft over Detroit on Christmas Day, 2009.

Nayef, who is seen as close to the country’s conservative clergy, has appeared skeptical of some of the king’s reforms, including plans to allow women some voting rights by 2015.

Nayef has also taken a hard line on the popular revolts that have toppled or threatened regimes across the Arab world. To stave off possible unrest in Saudi Arabia, Abdullah ordered expenditures of $130 billion on job creation, new housing and other social benefits.

The kingdom also sent troops into neighboring Bahrain to prop up its minority Sunni rulers, and Nayef warned Iran, an old nemesis, against stirring up protests among Saudi Arabia’s minority Shiite community.

The Obama administration’s support for pro-democracy movements across the Middle East and the toppling of several regimes in the region, especially the Egyptian government of Hosni Mubarak, have strained Saudi Arabia’s ties with the United States.

“U.S.-Saudi relations have been getting harder,” Alterman said, citing differences over Iraq, Iran and the Israel-Palestinian conflict, as well as the upheaval in the Arab world. “They think that the U.S. has been ineffective and harmful to its own interests and Saudi Arabia’s. I don’t see Nayef having the instinct to work hard to reverse that.”

Staff writer Joby Warrick in Uzbekistan contributed to this report.