A new flu virus suspected of killing up to 60 people in Mexico has the potential to become a pandemic, the World Health Organization's chief says.
Margaret Chan said the outbreak was a "serious situation" which needed to be followed closely.
Ms Chan cut short a visit to the US and returned to Geneva for urgent talks.
Health experts say tests so far seem to link the illnesses in Mexico with a new swine flu virus that sickened eight people in the southern US.
The WHO's new emergency committee is now meeting - though health officials say it may be premature to advise against travel to Mexico.
The committee will be discussing whether to declare a public health emergency, and whether to raise the global pandemic alert level.
The WHO says it does not know the full risk yet.
It is advising all member states to be vigilant for seasonally unusual flu or pneumonia-like symptoms among their populations - particularly among young healthy adults, who seem to be the most affected in Mexico.
Officials said most of those killed so far were young adults - rather than more vulnerable children and the elderly.
Precautions
Schools, museums and libraries have been closed across the capital region and people are being urged to avoid shaking hands or sharing crockery.
All public events have been suspended, an official said. Two previously sold-out soccer matches were played in empty stadiums to avoid potentially spreading the virus.
And in the US, the White House said it was watching the situation.
Dr Richard Besser, acting director of the Centers for Disease Control and Prevention (CDC), said that preliminary tests on seven out of 14 samples from patients in Mexico had matched the virus found in the US.
Eight people have fallen sick in the US - six in California and two in Texas - with a virus the WHO say was the same as the new strain of swine flu, designated H1N1.
All eight have recovered and only one was hospitalised.
The CDC said it planned to send experts to Mexico to help investigate the virus which has infected more than 1,000 people in the country.
Ms Chan confirmed the virus was an animal strain - a mixture of swine, human and avian flu viruses.
Swine flu is a respiratory disease which infects pigs. It does not normally infect humans, although sporadic cases do occur usually in people who have had close contact with pigs.
New Virus in Mexico - BBC
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