Oman’s ailing Sultan Qaboos bin Said, one of the Middle East’s longest serving rulers who maintained the country’s neutrality in regional struggles, died on Friday and the Gulf state’s high military council began the process to choose his successor, Reuters reported.

Three days of official mourning have been declared with flags flown at half-mast for 40 days, for the Western-backed Qaboos, 79, who had ruled since taking over in a bloodless coup in 1970 with the help of former colonial power Britain.

State news agency ONA did not give a cause of death, but Qaboos had been unwell for years and spent a week in Belgium undergoing medical treatment in early December.