World Health Organization (WHO) director-general Lee Jong-Wook of South Korea died today after surgery to remove a blood clot from the brain, the United Nations agency said.
Mr Lee (61) had been WHO chief since 2003 and was spearheading the organisation's fight against the global threat of bird flu.
"I am sorry to tell you that Dr Lee Jong-Wook, director-general of the WHO, died this morning," Spain's Health Minister Elena Salgado, told the opening meeting of the agency's annual assembly.
Her voice trembling, Ms Salgado described Mr Lee as an "exceptional person and an exceptional director-general.
"Under his leadership, the WHO has been strengthened and has been able to give an effective response to world [health] problems," she said before asking delegates from the 192 member states to observe two minutes' silence.
The annual assembly, which runs until Saturday, was then suspended for 30 minutes after which it was due to resume. "We must continue with our work," Ms Salgado said.
Mr Lee underwent an operation on Saturday to remove a blood clot at the Cantonal Hospital of Geneva. He had been taken ill suddenly in the afternoon.
The affable South Korean, who liked to pepper his press conferences with jokes, was a keen sportsman with no history of ill-health, officials said.
Mr Lee took office at the head of WHO as it was beginning to win its battle against SARS - Severe Acute Respiratory Syndrome - the highly contagious respiratory disease that killed hundreds of people around the globe after spreading from China.
The South Korean government had hailed Mr Lee's election, the first South Korean to head a major international organisation, as a significant step in the country's move into the international arena.