An outbreak of swine flu in Mexico and the United States is a quickly evolving situation that has "pandemic potential", the head of the World Health Organisation (WHO) said yesterday before an emergency meeting of influenza experts.

WHO Director General Margaret Chan said the North American outbreak of a never-before-seen virus was a very serious situation.

She called yesterday's emergency meeting to consider declaring an international public-health emergency over the outbreak, which is believed to have killed dozens of people in Mexico and sickened at least eight in the United State.

The experts are also expected to recommend whether WHO should raise its pandemic alert to a higher level.

At least 62 people have died from severe pneumonia caused by a flu-like illness in Mexico, according to the WHO. Some of those who died are confirmed to have a unique version of the A/H1N1 flu virus that is a combination of bird, pig and human viruses.

Mexico has closed schools, museums, libraries and theatres in a bid to contain the outbreak, which may have sickened about 1,000 people there.

"The situation is evolving quickly," Chan said at a telephone news conference in Geneva. "A new disease is by definition poorly understood.

"In the assessment of WHO, this is a serious situation which must be watched very closely."

"This is an animal strain of the H1N1 virus, and it has pandemic potential because it is infecting people," Chan said.

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

Public-health emergency

It is the first time Chan has convened such a crisis panel since the procedure was created almost two years ago, spokesman Gregory Hartl said.

The emergency committee may decide that the outbreak constitutes an international public-health emergency, and if so, whether WHO should consider measures, including travel advisories, trade restrictions and border closures.

The global body's flu pandemic alert level is now set to phase three - meaning there is no or very limited risk of a new virus spreading from human to human.

An increased alert level was considered likely, as initial evidence from the outbreak in Mexico indicates the virus has spread between people. Hartl said, however, that a decision might not be made yesterday.

Flu vaccine

The current seasonal flu vaccine is not believed to offer any protection against this new swine flu. But anti-viral drug Tamiflu appears to be fully effective against the H1N1 virus, and "Mexico and the United States already have large stocks of Tamiflu," Hartl said.

WHO, which has been monitoring the situation since Thursday, said 12 of the Mexican cases have been confirmed as genetically identical to a swine-flu virus detected in California.

US authorities said eight people were infected with swine flu in California and Texas, and all recovered.

"We do seem to have found incidents of the same illness, which is swine influenza A/H1N1, on both sides of the border in various locations," Hartl said.

WHO has sent experts to Mexico and asked countries to report any unusual flu outbreaks.

"It would be prudent for health officials within countries to be alert to outbreaks of influenza-like illness or pneumonia, especially if these occur in months outside the usual peak influenza season," Chan told reporters.

"Another important signal is excess cases of severe or fatal flu-like illness in groups other than young children and the elderly, who are usually at highest risk during normal seasonal flu," she said.

The fact that young, healthy adults appear particularly susceptible to the swine-flu outbreak has echoes of the 1918 flu pandemic, in which millions died.

Chan said it was too early to draw parallels at this stage.

"We are at the beginning of the outbreak here, and there are a lot of things that we still don't know," Hartl said.