Martti Ahtisaari at the Nobel Peace Prize award ceremony at Oslo City Hall on December 10, 2008. AFP

"It is with deep sadness that we have received the news of the death of President Martti Ahtisaari," the current President, Sauli Niinisto, said in a statement.

Serving as president from 1994 to 2000, the mediator negotiated an end to conflicts around the globe, from Indonesia to Kosovo and Namibia, earning him worldwide recognition and a Nobel Peace Prize in 2008, as part of a formidable career spanning over 40 years.

The former UN diplomat was credited with overseeing the 2005 reconciliation of the Indonesian government and Free Aceh Movement (GAM) rebels, bringing an end to a three-decade conflict that killed some 15,000 people.

The two sides described Ahtisaari as steely during the talks, but gifted with a sense of humour and warmth on the sidelines.

"I have enormous amounts of patience. I don't usually get angry, but I can be tough," he later said, adding that he felt the key to his success was an ability to understand people.

Ahtisaari also helped lead Kosovo down the path toward independence, even though his intense efforts failed to clinch an agreement with Serbia before Pristina unilaterally declared independence in 2008.

Ahtisaari withdrew from public life in September 2021 as he struggled with Alzheimer's disease.

Ahtisaari was celebrated around the world for brokering peace in conflict zones in Kosovo, Indonesia and Northern Ireland. He refused to accept that wars and conflicts were inevitable.

"Peace is a question of will. All conflicts can be settled, and there are no excuses for allowing them to become eternal," Ahtisaari said when he accepted the Nobel award in 2008.

His global fame boosted the image of Finland as it emerged from the shadow of the former Soviet Union.

At home, Ahtisaari was always a political outsider. But it was that very lack of political ties helped him to win Finland's first direct presidential election, in 1994, at the helm of the opposition Social Democrats.

As president, he supported Finland's European Union membership and encouraged voters to support the 1994 accession referendum, which passed with 57 percent support.

Opponents criticised his frequent travels - his nickname was "Travelling Mara", a common diminutive for Martti - and said he should focus more on domestic issues as Finland wallowed in recession triggered by the collapse of the Soviet Union, then its main trading partner.

In later years he strayed from his party's line by encouraging Finland to become a fully fledged NATO member. That took place years before Finland's eventually joined the alliance in 2023, in response to neighbouring Russia's invasion of Ukraine.

Agence France-Presse