Kuwait's 91-year-old ruler Emir Sheikh Sabah al-Ahmad al-Sabah will travel to the United States on Thursday for medical treatment after he underwent surgery this week, his office said.

Sheikh Sabah, who has ruled Kuwait since 2006, underwent surgery on Sunday that his office described as successful, for an unspecified condition.

"[Sheikh Sabah] will leave Kuwait at dawn...based on the advice of the medical team treating His Highness in order to complete treatment after a successful surgical procedure," said a statement from the emir's office published by state news agency KUNA.

The emir's designated successor Crown Prince Sheikh Nawaf al-Ahmed al-Sabah temporarily took over some of the ruler's constitutional duties on Saturday.

Last year, Sheikh Sabah was admitted to hospital in the United States while on an official visit there after suffering what his office described as a health setback in Kuwait in August. He returned to the Gulf Arab state in October.

Kuwait's stock index on Wednesday fell 3%.