Kuwait's Amir Sheikh Sabah al-Ahmad al-Jaber al-Sabah died Tuesday aged 91, plunging his country into mourning for a leader regarded by Gulf Arabs as a savvy diplomatic operator and a humanitarian champion.
The cabinet announced his brother and designated successor Crown Prince Sheikh Nawaf al-Ahmad al-Sabah as the new ruler, in a statement read on state television.
The parliamentary speaker tweeted that Sheikh Nawaf, 83, would be sworn in Wednesday.
Sheikh Sabah had ruled the wealthy oil producer and US ally since 2006, and steered its foreign policy for more than 50 years.
"With hearts filled with pain and sadness for the Kuwaiti people, the Islamic and Arab world and nations of the world, and with faith in the will of God, the cabinet mourns... Sheikh Sabah al-Ahmad al-Jaber al-Sabah who died in the United States on Tuesday," the statement said.
The Amir had been in hospital there since July following surgery in Kuwait that month.
Flags were flying at half-mast in Kuwait, which announced 40 days of mourning."Goodbye, Emir of Humanity," read a large banner on a street near the Kuwait Stock Exchange.
Kuwait Towers, a seaside landmark normally lit at night, went dark.
Condolences poured in from Arab leaders and several countries in the region announced mourning periods.
Sheikh Sabah sought to balance relations with Kuwait's bigger neighbours — forging close 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 blockade on Qatar, and made fundraising for humanitarian aid in Syria one of Kuwait's priorities.
Iranian Foreign Minister Mohammad Javad Zarif, in an Arabic-language tweet, lauded Sheikh Sabah for fostering "moderation and balance" in Kuwait and the region.
"Today we lost a big brother and a wise and loving leader... who spared no effort for Arab unity," said Jordan's King Abdullah, also on Twitter.
?Sabah kept strong ties with the United States, which led a coalition that ended Iraq's 1990-91 occupation of Kuwait and used the Gulf state as a launch pad for the 2003 Iraq invasion.
President Donald Trump earlier this month awarded the US Legion of Merit, Degree Chief Commander, to Sheikh Sabah in what the White House said was the first time the honour has been given since 1991.
The Amir's eldest son, Sheikh Nasser, accepted the award.
United Nations Secretary-General Antonio Guterres praised the Amir as "an extraordinary symbol of wisdom and generosity, a messenger of peace, a bridge builder".
Under Kuwait's constitution the crown prince automatically becomes Amir but assumes power only after taking an oath in parliament, for which elections are due this year.