| The current strain doesn't seem to be particularly dangerous, but H1N1 is notorious for mutating quickly, so it could indeed become a threat. |
Mr Fukuda also called for international vigilance as health experts wait to see whether the virus will turn into a worldwide pandemic.
Over 1,300 people are now thought to have contracted the virulent H1N1 swine influenza after it mutated into a form that spreads from human to human.
The Mayor of New York has confirmed that eight school children are suffering mild symptoms after becoming infected. And there have been at least 12 other confirmed cases in Texas, Ohio, California and Kansas.
The White House has declared a public health emergency but told the public "not to panic".
| If they say that, then it's a perfect time to panic ... |
Canada has become the third country to confirm human cases of swine flu with six people falling ill in Nova Scotia and British Columbia. Elsewhere in the world, suspected cases have been reported in France, Spain, Israel, New Zealand and the UK. In France, two people who had returned from Mexico with fevers are being monitored in regions near the port cities of Bordeaux and Marseille.
A 26-year-old Israeli man has also been admitted to hospital after returning from a trip to Mexico with flu-like symptoms.
In Auckland, 10 school children have tested positive for influenza after returning from Mexico. In the UK, two people have been admitted to a hospital in Scotland after returning from Mexico last week. They are said to have mild flu-like symptoms but their condition is not causing concern.
Mexican City Mayor Marcelo Ebrard said two more people have died of the virus, taking the death toll to 83.
All schools have been shut in Mexico City, the surrounding area and the central state of San Luis Potosi until May 6.
The WHO says it has a stockpile of the antiviral Tamiflu, which has proven effective against the virus, and is preparing a vaccine if needed.
The H1N1 strain of swine flu is usually only seen in pigs - but in humans can cause symptoms including fever and fatigue. The WHO says there is "zero evidence" that people are getting infected with the virus from exposure to pigmeat or pigs.
However, many countries say they are stepping up checks on pork imports from the region. Russia has banned meat imports from Mexico as well as from several US states and Central American countries.