Kuwait's 91-year-old ruler Sheikh Sabah al-Ahmad Al-Sabah left for the United States Thursday to undergo medical treatment, his office said, days after he had surgery for an undisclosed illness. The emir, who has ruled the oil-rich Gulf state since 2006, had been in hospital since the weekend. Sheikh Sabah "left the country today at dawn to go to the United States to complete his medical treatment", his office said in a statement cited by state news agency KUNA. Earlier, it had said he would make the journey "based on the advice of his medical team to complete treatment following the successful surgery". The statements did not reveal the nature of his illness, the type of surgery he had undergone in Kuwait, or what treatment was planned in the US. In September 2019, he underwent medical tests shortly after arriving in the United States, leading to a meeting with President Donald Trump being called off. The emir had his appendix removed in 2002, two years after having a pacemaker fitted. In 2007, he underwent urinary tract surgery in the United States. Under Kuwaiti law, when the emir is absent, crown prince Sheikh Nawaf al-Ahmad Al-Sabah, 83, the emir's half-brother, is appointed acting ruler. Sheikh Nawaf is an elder statesman who has held high office for decades, including the defence and interior portfolios. Sheikh Sabah argued last year for de-escalation in the Gulf as tensions surged between the US and its arch-foe Iran. He is widely regarded as the architect of modern Kuwait's foreign policy.