A new flu virus that has killed up to 81 people in Mexico could start a global epidemic, the World Health Organisation is warning as the disease spreads further in the United States.
Mexico's crowded capital of 20 million people, where most of the victims have died, hunkered down in fear of the swine flu and the government said it would isolate sick people if necessary.
It said the flu had probably killed 81 people, raising the likely death toll from 68, and that more than 1,300 people were believed to have been infected.
All schools in and around the sprawling capital and the central state of San Luis Potosi were ordered closed until May 6 and Health Minister Jose Angel Cordova called for all bars, clubs, stadiums, movie theatres, churches and other religious centers to be shut to limit further infections.
While all of the deaths so far have been in Mexico, the flu is spreading in the United States.
Eleven cases have been confirmed in California, Kansas and Texas, and eight schoolchildren in New York City caught a type A influenza virus that was likely to be the swine flu, health officials said.
The World Health Organisation declared the outbreaks a "public health event of international concern" and said they could cause a pandemic - a global epidemic of serious disease.
The last flu pandemic was in 1968 when "Hong Kong" flu killed about 1 million people globally.
A new pandemic would deal a major blow to a world economy already knocked into its worst recession in decades by the crisis in financial markets.
In Mexico City, parents canceled children's parties, bars were closed and residents stocked up on DVDs as people stayed home for the weekend to avoid contamination by a virus that has never been seen before.
"I think it's worse than they're telling us," said 35-year-old Lidia Diaz, sniffling and wearing a surgical mask as she headed to a clinic in the capital.
Mexican President Felipe Calderon issued an emergency decree giving the government special powers to run tests on sick people and order them to be isolated.
WHO Director-General Dr Margaret Chan urged all countries to boost their surveillance for any unusual outbreaks of influenza-like illness and severe pneumonia.
"It has pandemic potential because it is infecting people," Chan said in Geneva. "However, we cannot say on the basis of currently available laboratory, epidemiological and clinical evidence whether or not it will indeed cause a pandemic."
A British Airways cabin crew member was taken to a London hospital as a precaution after developing flu-like symptoms on a flight from Mexico City. It was the first such reported precautionary measure in Britain.
Hong Kong, Japan checks
As far away as Hong Kong and Japan, health officials stepped up checks of travelers with flu-like symptoms. The US Centers for Disease Control and Prevention said it was actively looking for new infections.
"We are worried and because we are worried we are acting aggressively on a number of fronts," the CDC's Dr Anne Schuchat told reporters. "The situation is serious."
In Mexico, 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.
The new flu strain - a mixture of swine, human and avian flu viruses - is still poorly understood.
A significant worsening of the outbreak could hit tourism and consumer spending in Mexico, already weakened by the global economic crisis and an army-led war on drug cartels.
No countries or global bodies have issued travel bans to Mexico but some countries alerted travelers to check websites for information on the flu outbreak.
The WHO says the virus from 12 of the Mexican patients is genetically the same as a new strain of swine flu, designated H1N1, seen in the people in California and Texas, who have recovered.