"He was a strong supporter of the deep and enduring partnership between our two countries forged almost seven decades ago," Obama said in a statement, adding that he learned of the crown prince's death with "great regret."

Sultan, who was 80 according to government records, served as the oil-rich Gulf state's defence minister for decades and had been in the United States since mid-June for medical treatment. He was operated on in July.

"As minister of defence 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," Obama said.

"On behalf of the American people, I extend my deepest condolences to King Abdullah, the royal family, and the people of Saudi Arabia."

Sultan had promoted large purchases of US arms, including tanks, helicopters, missiles and airborne warning and control systems.

"He will be missed," US Secretary of State Hillary Clinton said on a trip to Tajikistan, as she stressed Washington's enduring ties with the Arab nation.

"I offer my deepest condolences for this loss to King Abdullah and the Saudi people."