A new flu virus suspected of killing as many as 81 people in Mexico has the potential to become a pandemic, the World Health Organization's chief says.
Margaret Chan said the outbreak was a "health emergency of international concern" and must be closely monitored.
Health experts say tests so far seem to link the illnesses in Mexico with a swine flu virus in the southern US.
Several people have also fallen ill in the US, and the authorities there are watching the situation.
A top US health official said the strain of swine flu had spread widely and could not be contained.
Speaking after a meeting of the WHO's emergency committee, Mrs Chan said that "the current events constitute a public health emergency of international concern".
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HAVE YOUR SAYI work as a resident doctor in one of the biggest hospitals in Mexico City and sadly, the situation is far from 'under control'Yeny Gregorio Davila, Mexico CityRead more experiences
The WHO is advising all member states to be vigilant for seasonally unusual flu or pneumonia-like symptoms among their populations - particularly among young healthy adults.
Officials said most of those killed so far in Mexico were young adults - rather than more vulnerable children and the elderly.
The committee has not recommended declaring an international public health emergency and raising the global pandemic alert level, a move that could lead to travel advisories, trade restrictions and border closures.
New strain
At least some of the cases show a new version of the H1N1 swine flu sub-strain - a respiratory disease which infects pigs but only sporadically infects humans.
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The RC church has issued advice to its priests to help halt the spread of the fluH1N1 is the same strain that causes seasonal flu outbreaks in humans, but the newly-detected version contains genetic material from versions which usually affect pigs and birds.
The virus is spread through coughs and sneezes and through direct and indirect contact between people.
Mexican officials have confirmed 20 deaths from the virus and are investigating dozens more.
Schools, museums and libraries have been closed across the capital's region and people are being urged to avoid shaking hands or sharing crockery.
Hundreds of public events have been suspended and schools in the Mexico City area have been closed until 6 May.
Two previously sold-out soccer matches were played in empty stadiums to avoid potentially spreading the virus.
Health officials are isolating individuals suspected of having the virus and inspecting their homes.
The Roman Catholic Church in Mexico has recommended measures to avoid further contagion at Mass this Sunday.
Priests have been told to place communion wafers in the hands of worshippers rather than in their mouths and to suggest to the congregation that kissing or shaking hands be avoided during the service.
'Caution'
In the US, 11 people are now known to have been infected with the new strain - seven people in California, two in Texas, and two in Kansas.
There are also eight suspected cases in New York City after 200 students at a high school fell ill.
Specimens were taken from nine students, and eight were determined to be probable cases of swine flu, said city health commissioner Dr Thomas Frieden.
Those samples are now being examined by the US Centers for Disease Control and Prevention (CDC).
No children had required hospital treatment and many had fully recovered, said Dr Frieden, but the school could remain closed out of "an abundance of caution".
He urged people to maintain basic hygiene, such and covering their mouths when coughing and sneezing, washing hands regularly and keeping surfaces clean.
Dr Frieden said most people would not need to take antiviral medication if they fell ill, unless they had an underlying medical condition.
Hopeful sign
CDC officials have said that with cases arising in so many communities, containment is unlikely to be feasible.
There is currently no vaccine for the new strain.
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SWINE FLU Swine flu is a respiratory disease found in pigsHuman cases usually occur in those who have contact with pigsHuman-to-human transmission is rare and such cases are closely monitoredQ&A: Swine flu UK monitoring flu outbreak Tom Skinner of the CDC told the BBC that it was too early too tell how widespread the impact would be.
"We don't know how well or efficiently this virus is spreading and how easily it is going to be sustained in the human population."
He said it was not yet clear which side of the border the virus had originated.
But the US was likely to take "normal and routine" steps within the next few days to screen passengers coming into the US and to distribute information, he said.
The CDC plans to send experts to Mexico to help investigate the virus which has infected more than 1,000 people in the country.
The BBC science editor Susan Watts says the new strain is a classic "re-assortment" - a combination feared most by those watching for the flu pandemic.
