The World Health Organisation (WHO) has said the swine flu outbreak in Mexico and the US is a public health emergency of international concern and urged worldwide surveillance for any unusual outbreaks of influenza-like illness.
Governments around the world rushed today to check the spread of a new type of swine flu that has killed up to 81 people in Mexico.
Some 20 cases of swine flu were today confirmed in the United States as the White House stepped up efforts to monitor the outbreak, US officials said.
Dr Richard Besser, acting director of the Centers for Disease Control and Prevention, said at a White House briefing that all of the US cases have been mild.
US officials said President Barack Obama is being briefed regularly on the potential spread of the outbreak.
"At this point, a top priority is to ensure that communication is robust and that medical surveillance efforts are fully activated," John Brennan, assistant to the president for Homeland Security, said. He said that would enable rapid identification and notification of any new cases that might occur in the United States and Mexico.
Mexicans huddled in their homes while US hospitals tracked patients with flu symptoms and other countries imposed health checks at airports as the World Health Organisation warned the virus had the potential to become a pandemic.
The epidemic has snowballed into a monster headache for Mexico, already grappling with a violent drug war and economic slowdown, and has quickly become one of the biggest global health scares in years.
Mexico's tourism and retail sectors could be badly hit. A new pandemic would deal a major blow to a world economy already suffering its worst recession in decades.
In New Zealand, 10 pupils from an Auckland school party that had returned from Mexico were being treated for influenza symptoms in what health authorities said was a likely case of swine flu, although they added none was seriously ill.
"We are monitoring minute by minute the evolution of this problem across the whole country," Mexican President Felipe Calderon said as health officials counted suspected infections in six states from the tropical south to the northern border.
While all the deaths so far have been in Mexico, the flu is spreading in the United States. Eleven cases were confirmed in California, Kansas and Texas, and eight schoolchildren in New York City caught a type A influenza virus that health officials say is likely to be the swine flu.
The new flu strain, a mixture of various swine, bird and human viruses, poses the biggest risk of a large-scale pandemic since avian flu surfaced in 1997, killing several hundred people. A 1968 "Hong Kong" flu pandemic killed about 1 million people globally.
New flu strains can spread quickly because no one has natural immunity to them and a vaccine takes months to develop.
In Brussels, the European Commission said no cases of the new swine flu had been reported so far in Europe. "Until now we have no reported cases in Europe. We are following very closely the situation as it evolves," a spokeswoman for the European Union executive said.
In France, two people returning from Mexico who had flu-like symptoms were being tested, French public health director Didier Houssin told RTL radio.
A French health ministry spokeswoman said there were two unconfirmed cases but declined to give further details.
A British Airways cabin crew member was taken to hospital in London after developing flu-like symptoms on a flight from Mexico, but tests later cleared him of swine flu.
Mexican Health Minister Jose Angel Cordova said the swine flu had killed at least 20 and possibly as many as 81 people in Mexico, and more than 1,300 people were being tested for suspected infection. Most of the dead were aged 25 to 45, a worrying sign because a hallmark of past pandemics has been high fatalities among healthy young adults.
Additional reporting Reuters