President Xi Jinping expressed deep condolences and sympathy on Monday to the family of Kofi Annan and the United Nations, describing the diplomatic veteran as an active promoter of multilateralism.

Annan, 80, who served as UN secretary-general from 1997 to 2006, died on Saturday in a Swiss hospital.

Annan is an old friend of the Chinese people, and China's cooperation with the UN was comprehensively boosted during his term, Xi said in a condolence message sent to UN Secretary-General Antonio Guterres.

Xi said Annan, a world-renowned international statesman, promoted multilateralism and pushed for a stronger role for the UN, making remarkable contributions to world peace and development.

Xi hailed the former UN chief as an outstanding representative of the African people who came from and cared about Africa.

He said Annan pushed for greater attention and investment for Africa from global society.

The president also said the multilateralism promoted by Annan had special significance in a world faced with instability and uncertainty.

His efforts and contributions will be remembered by people in all countries, the president said.

World leaders, including Russian President Vladimir Putin, German Chancellor Angela Merkel and European Commission President Jean-Claude Juncker, have issued condolences following Annan's death.

Putin was quoted in a statement from the presidential press office as saying, "I sincerely admired his wisdom and courage as well as his ability to make balanced decisions even under the most dire and critical circumstances."

Merkel hailed Annan as an "exceptional statesman in the service of the global community".

Guterres described Annan as "a guiding force for good", saying that "in many ways, Kofi Annan was the United Nations", in a statement issued immediately after Annan's death.

Xinhua contributed to this story.