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 emergency committee, which can recommend declaring an international public health emergency and raise the global pandemic alert level - a move that could lead to travel advisories, trade restrictions and border closures - has been meeting.

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.

Public buildings have closed across Mexico City because of the outbreak

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.

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 hands of the worshippers rather than in their mouths and to suggest to the congregation that kissing or shaking hands be avoided during the service.

The US is watching the situation south of the border closely.

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.

SWINE FLU Swine flu is a respiratory disease found in pigs Human cases usually occur in those who have contact with pigs Human-to-human transmission is rare and such cases are closely monitored

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.