The US Centres for Disease Control and Prevention (CDC) says an unusual new flu virus has spread widely and cannot be contained, as the World Health Organisation urges governments to be on the alert.

"It is clear that this is widespread," the CDC's Dr Anne Schuchat told reporters.

"And that is why we have let you know that we cannot contain the spread of this virus."

The strain of swine flu is suspected of killing as many as 60 people in Mexico and infecting thousands more.

It is a new strain, and therefore poorly understood. The World Health Organisation (WHO) says it does not know the full risk yet.

The organisation has held an emergency meeting to discuss the outbreak.

Its director-general, Margaret Chan, says the deadly new form is serious and does have pandemic potential.

"It has pandemic potential because it is infecting people," WHO Director-General Margaret Chan said in Geneva.

"However, we cannot say on the basis of currently available laboratory, epidemiological and clinical evidence whether or not it will indeed cause a pandemic."

The WHO is advising all countries 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.

The CDC's acting director, Dr Richard Besser, says it seems humans are transmitting the virus.

"There are things that we see that suggest that containment is not very likely, in that we are seeing cases in Texas and we're seeing cases in San Diego without any connection between them, which makes us think that there's been transmission from person to person," he said.

The Mexican government says there will be a mass vaccination campaign, and in the meantime it has closed all schools and universities.

CDC officials are assisting public health authorities in Mexico to test additional specimens and providing epidemiological support as part of a WHO team.

The centres have also dispatched teams in southern California, where several cases were reported.

Health leaders in the US, Mexico, Canada and at the WHO say they are communicating frequently, and state and local US health authorities are conducting investigations.

There is no vaccine to protect humans from swine flu, only to protect pigs, according to the CDC.

Dr Schuchat says measures are being taken to produce a vaccine against the virus if necessary, but cautions that it usually takes "months" to produce a vaccine.

"We're not going to have large amounts of vaccine tomorrow," she warned.

- ABC/BBC/AFP/Reuters