Former United Nations Secretary General Kofi Annan has died aged 80, his foundation said on Saturday.

Annan died in hospital in Bern, Switzerland, in the early hours of Saturday, two close associates of Annan told Reuters news agency.

The UN’s migration body said on Twitter: “Today we mourn the loss of a great man, a leader, and a visionary: former UN Secretary General [Kofi Annan]. A life well lived. A life worth celebrating.”

Former Prime Minister Tony Blair paid tribute to a “good friend” and a “great diplomat”.

Blair, whose time in office coincided with Annan’s, said: “I’m shocked and distressed to hear the news about Kofi. He was a good friend whom I saw only weeks ago.”

Blair, who clashed with Annan in 2003 over Britain’s decision to invade Iraq with the US, a move which the then secretary general denounced as illegal, said Annan was “widely respected and will be greatly missed”.

“Kofi Annan was a great diplomat, a true statesman and a wonderful colleague,” he added.

“My deepest sympathy to Nane and his family.”

While former PM Gordon Brown and his wife Sarah said: “A leader of leaders, a titan amongst world statesman, a wonderful humanitarian and the most compassionate and caring of individuals.

“Kofi Annan will be sorely missed.”

Theresa May said that she was sad to learn of Annan’s death and that her thoughts were with his family.

The Ghanian-born diplomat was head of the UN in 1997 to 2006.

In 2001, Annan and the United Nations were jointly awarded the Nobel Prize for Peace.

He joined the UN system in 1962 as an administrative and budget officer with the World Health Organization in Geneva, Switzerland, according to an official biography.

He held positions within the UN’s emergency force, its high commission for refugees and senior posts in human resources and staff security before becoming secretary general in 1997.

Upon leaving the UN, Annan founded an eponymous foundation dedicated, its website said, to helping mobilise “political will to overcome threats to peace, development and human rights.”

Annan is survived by his wife, Nane, and their three children.