Lee had led the WHO's work to combat the spread of bird flu, KAZINFORM refers to CNN.

Lee, who spearheaded the U.N. health agency's fight against the spread of bird flu and preparing for a potential human influenza pandemic, died Monday morning, said South Korea's Ambassador to Switzerland, Park Won-hwa. Lee was 61.

It is a "very sad bit of news," Elena Selgado, Spain's health minister and president of the World Health Assembly, said at the opening of the annual meeting of the 192 members of WHO.

Lee "was an exceptional person and an exceptional director-general," Selgado said.

Lee, who took over as director-general of WHO in 2003, underwent emergency surgery for a blood clot in his brain at the weekend in the Cantonal Hospital of Geneva.

He fell ill Saturday afternoon while attending an official function. He was taken to a hospital and medical staff operated immediately.

WHO had already said Lee would be hospitalized for "some time" and would be unable to attend the World Health Assembly, the annual meeting of WHO members which started Monday.

Lee had worked for WHO for 23 years, including time served in regional posts.