Former United Nations secretary-general and Nobel Peace Prize laureate Kofi Annan has died at the age of 80, his foundation said on Saturday.

Mr Annan, a Ghanaian national, died in hospital in Bern, Switzerland, in the early hours of this morning.

In Geneva, the Kofi Annan Foundation announced his death with "immense sadness" after a short illness, saying he was surrounded in his last days by his second wife Nane and children Ama, Kojo and Nina.

Mr Annan served two terms as UN Secretary-General in New York from 1997-2006 and retired in Geneva and later lived in a Swiss village in the nearby countryside.

His ten-year-old foundation promotes good governance and transformation of African agriculture.

Former President Mary Robinson, a friend of Mr Annan from her time working as UN High Commissioner of Human Rights, told RTÉ News that he died "living the life he believed in." 

"Kofi was not well. And he was pushing himself. It was really remarkable to see his determination to give his all for peace and democracy and human rights in Zimbabwe. 

"On the way back to Geneva, he got ill and he really never recovered."

President Michael D Higgins has said Mr Annan left "an indelible legacy on the global community".

He said Mr Annan would in particular be remembered for his tireless efforts to increase the commitment of governments and businesses alike in efforts to eradicate poverty and violent conflict.

He added that his wisdom, empathy, humour and insights would be greatly missed.

Taoiseach Leo Varadkar described Mr Annan as "a true multilateralist, at a time when we need multilateralism, and a tireless advocate for world peace."

Current Secretary General Antonio Guterres said Mr Annan had been "a guiding force for good."

"In many ways, Kofi Annan was the United Nations. He rose through the ranks to lead the organisation into the new millennium with matchless dignity and determination," Guterres, whom Annan had chosen to head the UN refugee agency, said in a statement.

As head of UN peacekeeping operations, Mr Annan was criticised for the world body's failure to halt the genocide in Rwanda in the 1990s.

As UN boss he was linked to peace efforts to reunite the divided island of Cyprus.

He submitted a reunification blueprint for Cyprus which was rejected in a referendum by Greek Cypriots in 2004.

He staunchly opposed the US-led invasion of Iraq in 2003 and later served as the first UN envoy at the start of Syria's war, but quit after world powers failed to fulfil their commitments, saying: "I lost my troops on the way to Damascus".

"The UN can be improved, it is not perfect but if it didn't exist you would have to create it," he told the BBC's HARDTalk during an interview for his 80th birthday last April, recorded at the Geneva Graduate Institute where he had studied.

"I am a stubborn optimist, I was born an optimist and will remain an optimist," Mr Annan added.

He worked as chair of The Elders, a group of independent leaders who use their collective experience and influence for peace, justice and human rights worldwide. The group was founded by Nelson Mandela in 2007.

In a statement, the Elders said they were deeply saddened at his passing.

Deputy Chair of the Elders Gro Harlem Brundtland said Mr Annan was a "strong and inspiring presence".

It is with immense sadness that the Annan family and the Kofi Annan  Foundation announce that Kofi Annan, former Secretary General of the  United Nations and Nobel Peace Laureate, passed away peacefully on  Saturday 18th August after a short illness...

— Kofi Annan Foundation (@KofiAnnanFdn) August 18, 2018

UN High Commissioner for Human Rights, Zeid Ra'adal-Hussein, paid tribute to Mr Annan as "humanity's best example, the epitome, of human decency and grace".
Mr Zeid, who has criticised major powers and other countries during his four-year term that ends later this month, said that whenever he felt "isolated and alone politically", he would go for long walks with Mr Annan in Geneva.

"When I told him once how everyone was grumbling about me, he looked at me like a father would look at a son and said sternly: 'You're doing the right thing, let them grumble'. Then he grinned".

Ghana has declared a week of mourning after Mr Annan's death.

Ghanaian President Nana Akufo-Addoon has hailed him as a consummate international diplomat who had brought immense pride to his country.

He ordered Ghana's national flag to be flown at half-mast across the country and in all of the country's diplomatic missions from Monday for one week.

Mr Akufo-Addo said the government and people of Ghana were saddened by the news of the death of "one of our greatest compatriots".

"Consummate international diplomat and highly respected former Secretary General of the United Nations, Mr. Kofi Annan was the first from sub-Saharan Africa to occupy this exalted position. He brought considerable renown to our country by this position and through his conduct and comportment in the global arena," an official statement said.

"He was an ardent believer in the capacity of the Ghanaian to chart his or her own course onto the path of progress and prosperity."