RIYADH (AFP) - Saudi Crown Prince Sultan bin Abdul Aziz has died, the royal palace announced on Saturday, leaving his brother Prince Nayef in line to succeed King Abdullah at a time of turmoil in the Arab world. The crown prince, aged around 86, served as the oil kingpins defence minister for nearly five decades but had been in the United States since mid-June for medical treatment. He was operated on in July. A half-brother of King Abdullah, Sultan had spent long periods abroad for undisclosed medical treatment. A Western diplomat, speaking to AFP on condition of anonymity, said Prince Sultan had been hooked up to life support systems at New York Presbyterian hospital and was declared 'clinically dead more than a month ago. Condolences flooded into the kingdom from world leaders as news of the princes death was confirmed. US Secretary of State Hilary Clinton, speaking on a trip to the Central Asian nation of Tajikistan, said the prince would be 'missed, stressing Washingtons enduring ties with the oil-rich Gulf state. I offer my deepest condolences for this loss to King Abdullah and the Saudi people, she said. He will be missed. In Jordan, King Abdullah II opened the World Economic Forum in the countrys Dead Sea resort town with a minute of silence in the late princes honour, hailing him as a champion of the Arab and Muslim cause. Britains Prime Minister David Cameron said he was saddened to hear of Sultans death. He had many friends in this country, and we have all benefited from his wisdom and expertise in international affairs over his long years of service, the prime minister said. Syrian President Bashar al-Assad, despite sprains in ties with Riyadh over his security forces deadly crackdown on anti-regime protests, also sent condolences. He sent a message of condolences to Saudi King Abdullah over the death of his brother and crown prince, in which he expresses in the name of the Syrian people sincere condolences and sympathy, Syrias state news agency SANA said. Sultan died while King Abdullah, himself, 87, remains in hospital in Riyadh, a week after he had an operation on his back. State television aired footage Wednesday of King Abdullah in hospital as he received royal dignitaries. The king underwent an operation in November for a debilitating herniated disc complicated by a haematoma, or blood clot, that put pressure on his spine and underwent further surgery to repair several vertebrae. The latest operation was aimed at repairing a slackening ligament near the third vertebra. State media said the surgery was successful. The advanced age and failing health of the king and of his half-brothers in line to succeed him have raised concerns about the future of the oil giant in the face of the turmoil rocking the Arab world. The Al-Saud family has ruled Saudi Arabia ever since the kingdoms establishment in 1932 and under its rules of succession the throne passes in turn to the sons of its founder Abdul Aziz, all of whom are now elderly. Prince Nayef, who becomes crown prince on the death of Prince Sultan, is 78. Another half-brother of King Abdullah, he holds the interior portfolio. Prince Nayef was named second deputy premier in March 2009, filling a potential void in the succession lineup at a time when unprecedented Arab uprisings have unseated three autocratic leaders since January. Except for a few demonstrations by the Shia minority in eastern Saudi Arabia, the Sunni-dominated absolute monarchy has been largely spared the wave of pro-democracy protests sweeping other Arab countries. But it has felt increasingly threatened by the growing influence in the region of its Shia arch-rival Iran, which Riyadh accuses of sowing sectarian tensions. In March, Riyadh spearheaded a military intervention by Sunni-ruled Gulf states in Bahrain as its Sunni ruling family crushed month-long pro-democracy protests led by the island states Shia majority. Prince Sultans funeral is to be held on Tuesday after his body has been repatriated from the United States. Sultan had battled colon cancer since 2004, going to Switzerland and then the United States to seek treatment, according to diplomats. His long illness and absences abroad held up important government decisions while raising questions about how the monarchy would take shape in the next generation of the Al-Saud family. Then King Faisal assigned him the defence portfolio in 1962, and he became part of a handful of key princes, including Abdullah and Sultans full brothers Prince Nayef and Prince Salman, who have run the country for over four decades. In 1982, his full brother king Fahd appointed Sultan second deputy prime minister, making him second in line to the succession after then-crown prince Abdullah. Staff Reporter from Islamabad adds: The government of Pakistan on Saturday announced a three-day national mourning as a token of respect to the Saudi Crown Prince who breathed his last. The national flag will fly at half-mast as a mark of respect, a spokesman to the Prime Minister said. Earlier, President Asif Ali Zardari and Prime Minister Syed Yousuf Raza Gilani expressed deep sadness on the passing away of Sultan bin Abdul Aziz Al Saud, Crown Prince, First Deputy Premier and Minister of Defence of Saudi Arabia. The Pakistani leadership has conveyed condolence on behalf of the people of Pakistan to Khadim Al-Harmain Al Sharifain, King Abdullah bin Abdul Aziz Al-Saud, the royal family and the brotherly people of Saudi Arabia. Prince Sultan who was highly respected in Pakistan, will always be remembered here as a dear friend and brother. Chief of Army Staff General Ashfaq Parvez Kayani has expressed his deep condolences over the sad demise of Saudi Crown Prince Sultan Bin Abdel Aziz. According to ISPR on Saturday, the COAS in his condolence said that the Crown Prince Sultan Bin Abdel Aziz was a great friend of Pakistan and his services in boosting ties between armies of both the countries are highly commendable. PML-N President Nawaz Sharif, Punjab Chief Minister Shahbaz Sharif also expressed their heartfelt condolences over the sad demise of the Saudi Crown Prince.