An official at the World Health Organization (WHO) says the swine flu virus that has killed dozens in Mexico could mutate into a "more dangerous" strain.

WHO acting assistant-general for health, security and the environment Keiji Fukuda made the comment on Sunday.

He said it's quite possible for the virus to evolve, adding that if it does, it could become more dangerous.

Fukuda said, however, that the world is better prepared than in the past to prevent the spread of such viruses.

The official said the WHO has begun preparations to produce vaccines to counter the virus while stressing that there is no evidence of people getting ill from exposure to pork or pigs.

Fukuda confirmed eleven cases in the United States and 16 in Mexico, but stressed that the WHO was still trying to confirm the exact death toll resulting from the flu strain in the Latin American country.

In Seoul, the government has launched efforts to guard against the spread of the deadly flu in the nation.

Swine flu is a respiratory disease of pigs caused by type A influenza that regularly causes outbreaks of influenza among pigs. Human infections with swine flu do occur, but cases of human-to-human spread of swine flu viruses is rare.