Sultan Qaboos bin Said Al Said of Oman, the Arab world's longest-serving ruler, has died aged 79.
"With great sorrow and deep sadness... the royal court mourns His Majesty Sultan Qaboos bin Said, who passed away on Friday," a court statement said.
Last month he returned home after undergoing medical checks and treatment in Belgium. There were reports he was suffering from cancer.
Sultan Qaboos was unmarried and had no heir or designated successor.
Three days of national mourning have been declared.
The sultan deposed his father in a bloodless coup with British support in 1970. Using its oil wealth, he then set Oman on a path to development.
According to the sultanate's Basic Statute, the Royal Family Council - comprising about 50 male members - should choose a new sultan within three days of the throne falling vacant.
I send my condolences to the people of Oman, and I hope that the Omani royal family will decide on a successor sooner rather than later. He has done quite a lot to improve Oman's economy, and his foreign policy is certainly something to be lauded, especially since he was able to play a big role in shaping that nuclear deal between the USA and Iran.
May Qaboos rest in peace.