UN Secretary-General Ban Ki- moon will be writing to King Salman of Saudi Arabia to say that it was with deep sadness that he learned of the passing of Prince Saud bin Faisal bin Abdulaziz Al-Saud, who was the foreign minister of Saudi Arabia, a UN spokesman told reporters here Friday.

Al-Saud was the world's longest serving foreign minister when he stepped down, the spokesman said at a daily news briefing.

"The secretary-general will say that Prince Saud Al-Faisal was a friend of the United Nations, and the secretary-general will remember with fond appreciation his meetings and interactions with the prince," Dujarric said.

The former Saudi foreign minister, who was born in 1940, died on Thursday in the United States, two months after he retired following 40 years in the job.

Prince Saud, who was appointed in 1975, was the world's longest serving foreign minister when he was replaced on April 29 by Adel al-Jubeir, the then ambassador to Washington.