Kuwait’s ruling Emir Sheikh Sabah al-Ahmad al-Sabah has died, his office said in a statement read out on state television on Tuesday.

Sheikh Sabah, 91, had ruled the Gulf Arab oil producer and U.S. ally since 2006 and steered its foreign policy for more than 50 years. His designated successor is his brother, Crown Prince Sheikh Nawaf al-Ahmad al-Sabah.

“With the utmost sadness and grief for the Kuwaiti people, the Islamic and Arab world and people of friendly nations, the Emiri Diwan mourns the death of Sheikh Sabah al-Ahmad al-Jaber al-Sabah, the emir of Kuwait,” his office said.

Sheikh Sabah sought to balance relations with Kuwait’s bigger neighbours -- forging the closest ties with Saudi Arabia, rebuilding links with former occupier Iraq and keeping open dialogue with Iran. He tried to mediate in a Gulf dispute that saw Riyadh and its allies impose a boycott on Qatar.