The swine flu virus that has killed more than 80 people in Mexico may mutate into a "more dangerous" strain, the World Health Organisation has warned.
"It's quite possible for this virus to evolve... when viruses evolve, clearly they can become more dangerous to people," said Keiji Fukuda, of the global health watchdog.

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.

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".

Sky US correspondent Greg Milam said: "It's important to realise that those affected have only had mild symptoms, and all have recovered or are recovering.

"But the authorities do believe that this outbreak will get worse.