The UN Migration Agency tweeted: "Today we mourn the loss of a great man, a leader, and a visionary."

Annan was the first black African to take up the role of the world's top diplomat, serving two terms from 1997 to 2006. He later served as the UN special envoy for Syria, leading efforts to find a peaceful solution to the conflict.

Annan was awarded the Nobel Peace Prize jointly with the UN in 2001 "for their work for a better organized and more peaceful world". He was married with three children.