GENEVA - The outbreak of a new swine flu virus that has killed up to 60 people in Mexico and crossed into the US has "pandemic potential," the head of the World Health Organisation warned Saturday.

Transmitted from human to human, "a new virus is responsible" for around 1,000 cases reported in the Americas, WHO Director-General Margaret Chan said, adding: "It is a serious situation which needs to be closely followed."

How the disease will evolve is "unpredictable," the Chinese health expert added in a telephone press conference, urging countries in the region and elsewhere to "increase vigilance".

"This virus clearly has a pandemic potential," said Chan, who first came to wider attention with her handling of a bird flu epidemic which mutated and killed six people in Hong Kong in 1997.

The WHO went on high alert Friday, with Chan cutting short a visit to the United States to coordinate the global response from Geneva with experts dispatched to Mexico to aid authorities on the ground.

There, the main outbreak has hit the capital, Mexico City, with between 18 and 20 confirmed deaths due to the virus, according to WHO and Mexican figures, respectively.

On Saturday, pedestrians in the sprawling metropolis wore surgical masks distributed free at metro stations -- after authorities warned against underground travel, which increases the risk of contagion.

While no new deaths from the disease were confirmed, about 40 more fatalities are still being tested for links.

San Luis Potosi in central Mexico, where three people died, and Mexicali on the US border, have also suffered cases of the disease.

Some 1,000 patients are under observation, with schools, colleges, theatres and museums closed in the capital. Two football games will be played Sunday without spectators.

Meanwhile, 75 students at the private St. Francis Preparatory School in Queens in New York City have been undergoing tests after showing flu-like symptoms, according to CNN.

Several of the students recently visited Mexico, the New York Times reported Saturday.

Investigators descended on the school Friday after officials said symptoms "were consistent with a strain of swine flu that has swept Mexico City."

The WHO said Friday that the virus detected in 12 fatal Mexican cases is genetically identical to that which affected eight people in the US border states of California and Texas, though all of those recovered.

On Saturday, Chan stressed that there were no reports of similar outbreaks in other parts of the world, but underlined: "It has pandemic potential as it is infecting people."

A spokesman for the UN body, Thomas Abraham, told AFP that "the most worrying fact is that it appears to transmit from human to human."

Dave Daigle, of the US Centers for Disease Control, which is working closely with the WHO, said a bird flu strain, two swine flu strains and a human strain had combined for the first time.

These features, along with the fact that unusually young healthy adults have fallen victim, and not the very old or very young, have given rise to fears of a serious epidemic, if not a pandemic.

According to the WHO, pigs have already been factors in the appearance of two previously unknown diseases that gave rise to pandemics in the last century.

Mexico City authorities initially announced a mass vaccination campaign using regular human flu vaccines, but later said that the WHO had advised them that it was better to use antiviral medicines.

"It should be possible to produce a vaccine since this virus has been identified," WHO spokesman Abraham said. "But it will take some time."

In Canada, where the WHO has sent samples for lab testing, health authorities are also taking extra precautions.

A pandemic occurs when there is a new virus to which few people have resistance; the virus is easily transmissible and sustainable within a population, and causes severe illness.

- AFP /ls