Kuwait's ageing ruler Emir Sheikh Sabah al Ahmad al Sabah flew to the US on Thursday to seek further medical care after undergoing surgery in the Persian Gulf oil producer.

The royal court said Sheikh Sabah, 91, was leaving the country "based on advice from the medical team treating His Highness to complete his treatment after a successful surgical procedure".

The surgery took place on Sunday.

The statement — attributed to Sheikh Ali Jarrah al Sabah, the country's royal court minister — did not elaborate or say where in the US the emir will receive treatment.

Crown Prince Nawaf al Ahmad al Sabah, 83, has assumed some of Sheikh Sabah's powers temporarily.

Under Kuwaiti law, when the emir is absent, Sheikh Nawaf — the emir's half-brother — is appointed acting ruler of the Opec producer.

Sheikh Nawaf is an elder statesman who has held high office for decades, including the defence and interior portfolios.

Sheikh Sabah took power in 2006. He underwent medical tests last September shortly after arriving in the US, leading to a meeting with US 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 US.

Political analysts say Sheikh Sabah’s sudden surgery could renew a power struggle within Kuwait’s ruling family.