Sabah Al-Ahmad Al-Jaber Al-Sabah, The Amir of Kuwait, passed away on Sept. 29. The Kuwait monarch was 91 years old and had reigned since January 2006.

Queen Elizabeth II joined the global mourning of the well-liked leader on Sept. 30. In a message shared to the Royal Family's social media pages, the Queen called the late royal a "deeply valued" friend to the UK and offered her condolences to the people of Kuwait.

Queen Elizabeth II's message on HRH The Amir of Kuwait's death

The Queen's message of condolence was accompanied by a cheerful photo of her with The Amir during his 2012 State Visit, recalled in Her Majesty's letter. It was addressed to the Amir of Kuwait who succeeded to the throne on Sept. 30.

The message reads: "I was saddened to hear of the death of His Highness Sheikh Sabah Al Ahmad Al Jaber Al Sabah. Your distinguished brother devoted his life to the service of the state of Kuwait and especially its relationship with its allies and friends.

"He will long be remembered by all who work for regional stability, understanding between nations and between faiths, and for the humanitarian cause. I have deeply valued his friendship to the United Kingdom and his memorable State Visit in November 2012.

"I offer Your Highness my sincere condolences. I offer also my sympathy to the people of Kuwait. May the long history of close companionship between our two families continue," the message concluded, signed off - Elizabeth R.

The 5th Amir of Kuwait dies, age 91

The Amir of Kuwait's death was announced on state television on Tuesday, Sept. 29. He's thought to have died in Rochester, Minnesota, where he'd been recovering after surgery at the Mayo Clinic in the city.

The former monarch's half-brother, Nawaf Al-Ahmad Al-Jaber Al-Sabah, has succeeded him as the 6th Amir of the State of Kuwait. Queen Elizabeth would have learned of the Amir's passing at the Sandringham Estate, where she's been staying since mid-September.

The Queen, The Amir of Kuwait and Prince Philip in 2012.