The World Health Organisation raised the pandemic threat level from swine flu to phase 5 tonight as the virus spread and killed the first person outside Mexico, a toddler in Texas.
Nearly a week after the H1N1 virus, or swine flu first emerged in California and Texas and was found to have caused deaths in Mexico, Spain reported the first case in Europe of swine flu in a person who had not been to Mexico, illustrating the danger of person-to-person transmission.
Phase 5 is the WHO's second highest level of warning that a pandemic, or global outbreak of a serious new illness, is imminent.
"It is clear that the virus is spreading and we don't see evidence of it slowing down at this point," Dr Keiji Fukuda, WHO acting assistant director-general, told a news briefing in Geneva.
In Mexico, where up to 159 people have died from the virus and around 1,300 more are being tested for infection, people struggled with an emergency that has brought normal life virtually to a standstill.
US officials said that a 22-month-old boy had died in Texas - the first confirmed US swine flu death - but they added that he was on a family visit from Mexico, where up to 159 flu fatalities have been recorded.
Dr Richard Besser, acting head of the US Centres for Disease Control and Prevention, said the country now had 91 confirmed cases in 10 states from New York to California.
"We're going to find more cases. We're going to find more severe cases and I expect that we'll continue to see additional deaths," Dr Besser said.
President Barack Obama, who yesterday asked for $1.5 billion in emergency funding to fight the disease, said the death showed it was time to take "utmost precautions" against the virus.
France said it would seek a European Union ban tomorrow on flights to Mexico. Argentina and Cuba have already stopped flights from Mexico.
The EU, the United States and Canada have advised against non-essential travel to the popular tourist destination, as nearly all the cases so far, in Canada, New Zealand, Israel and Spain, have been linked to travel from Mexico.
The WHO said there was no plan now to call an emergency committee meeting to raise the pandemic alert level, which could take place if it were confirmed that infected people in at least two countries were spreading the disease to other people in a sustained way.
Dr Fukuda, who earlier said the outbreak may end up as a "very mild pandemic", said the report of the Spanish case involving a person who had not been to Mexico suggests the virus is spreading more easily among people.
"There are cases which are occurring in people who have not traveled," he told a news briefing in Geneva. "What we are looking for overall is whether we see many kinds of those infections occurring that suggest transmission is occurring independent of travel."
Spain's El Mundo newspaper said the Spanish patient had recent contact with someone who had visited Mexico.
H1N1 swine flu poses the biggest risk of a large-scale pandemic since avian flu re-emerged in 2003, killing 257 people of 421 infected in 15 countries. In 1968 a "Hong Kong" flu pandemic killed about one million people globally, and a 1957 pandemic killed about 2 million.
The new strain contains genetic material from avian, swine and human viruses and appears to have evolved the ability to pass easily from one person to another.
It cannot be caught from eating pork products but Egypt ordered all its pigs to be slaughtered and some countries, led by Russia and China, have banned US pork imports.
The WHO said today it had not been told officially of any such bans, and the EU and Japan said they would not follow suit.
In the United States, where pork producers have voiced outrage over the trade bans, officials began referring to the outbreak as the 2009 H1N1 flu.
Mr Obama's newly confirmed Health and Human Services Secretary Kathleen Sebelius held her first news conference today seeking again to reassure the public. “We are determined to fight this outbreak and do everything we can to protect the health of every American," she said.
The outbreak has deeply affected life in Mexico and ravaged tourism, a key earner. Mexico City was unusually quiet, with schools closed. Many parents took their children in to work.
All Mayan and Aztec pyramid ruins, dotted through central and southern Mexico, were closed until further notice.
Mexican Health Minister Jose Angel Cordova said more than 1,300 people were in hospitals, some of them seriously ill, out of a total of about 2,500 suspected cases.
"In the last few days there has been a decline (in cases)," he said. "The death figures have remained more or less stable."
Seasonal flu kills 250,000 to 500,000 people in a normal year, including healthy children in rich countries.