Kuwaiti Emir Sheikh Sabah al-Jaber al-Ahmad al-Sabah died on Tuesday at the age of 91, state media announced.
"With great sadness and sorrow we mourn to the Kuwaiti people, the Arab and Islamic nations, [...] the death of the late His Highness Sheikh Sabah al-Ahmad Al-Jaber Al-Sabah, Emir of the State of Kuwait," read a royal court statement carried by the official Kuwait News Agency.
The Kuwaiti Cabinet announced a 40-day national mourning and the closure of government departments for three days starting on Tuesday, according to the news agency.
On Sept. 14, the Kuwaiti Cabinet announced that the health of the emir, who led the country since Jan. 29, 2006, was "improving."
On July 23, al-Sabah headed to the US to continue treatment, based on the advice of the medical team treating him after a successful surgery.
In a statement, Turkish President Recep Tayyip said he received the news of the emir's demise with "great sadness".
"On behalf of the Turkish nation, I convey my sincere condolences to the friendly and brotherly people of Kuwait."
Qatari Emir Sheikh Tamim bin Hamad al-Thani expressed grief over the demise of his Kuwaiti counterpart, saying: "We lost a great leader who was characterized by wisdom and moderation."
Qatar announced three days of mourning over the death of the Kuwaiti Emir.
Egyptian President Adel-Fattah al-Sisi also offered condolences and declared a three-day mourning.
In Jordan, the Royal court described his death as "a great loss".
In a condolence message, Palestinian President Mahmoud Abbas said al-Sabah was "a big brother to the Palestinian people" and that he always stood by the Palestinian cause.
Lebanese President Michel Aoun said al-Sabah was "an example of generosity, moderation and wisdom."
Iraqi Prime Minister Mustafa al-Kadhimi said: "The [Arab] nation and the region lost a man who spared no efforts to support the region's security and stability."
Mohammad Abdelsalam, the Yemeni Houthi group spokesman, said: "On the day of the departure of the emir of Kuwait, we do not forget his stance in support of the [Yemeni] peace consultations [...] In every meeting with him, he expressed his love for Yemen and keenness to extinguish the fire of war."
* Ahmed Asmar, Mahmoud Barakat and Ali Murat Alhas contributed to this report from Ankara
