"We are taking this very seriously," she said, adding that the US was working with the WHO and other countries to "develop a strategy" to prevent the spread of the virus.
Earlier, EU Health Commissioner Androulla Vassiliou called on people to think carefully before going to places where people had been infected.
"Personally, I would try to avoid non-essential travel to the areas which are reported to be in the centre of the clusters in order to minimise the personal risk and to reduce the potential risk to spread the infection to other people," she said.
President Barack Obama, in a speech to US scientists, said the cases were a cause for concern but not alarm.
He said a public health emergency - declared after cases were found in New York, California, Texas, Kansas and Ohio - was a "precautionary tool" to ensure that all the necessary resources are available "to respond quickly and effectively".
Health experts say the virus comes from the same strain that causes seasonal outbreaks in humans. But they say this newly-detected version contains genetic material from versions of flu which usually affect pigs and birds.
The Spanish flu pandemic remains the most devastating outbreak of modern times - infecting up to 40% of the world's population and killing more than 50m people, with young adults particularly badly affected
Asian flu killed two million people. Caused by a human form of the virus, H2N2, combining with a mutated strain found in wild ducks. The elderly were particularly vulnerable
An outbreak first detected in Hong Kong, and caused by a strain known as H3N2, killed up to one million people globally, with those over 65 most likely to die
There is currently no vaccine for this new strain, but severe cases can be treated with antiviral medication.
The WHO said it had not made any recommendations about travel restrictions.
But spokesman Gregory Hartl said the fact that an emergency committee meeting had been brought forward by a day showed the urgency with which the organisation was treating the issue.
The committee could decide to raise the pandemic level from its current level of three, if it believes the virus is showing a sustained ability to pass from human to human.
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, a characteristic of past pandemics.
A top US health official warned that there could be worse to come.
"From what we understand in Mexico, I think people need to be ready for the idea that we could see more severe cases in this country and possibly deaths," Richard Besser, acting head of the US Centers for Disease Control and Prevention, told ABC television.
The BBC talks to people in Mexico City about the flu outbreak.
In the Mexican capital, schools, bars and public buildings remained closed and many people were staying indoors.
Mexican officials said they would keep schools closed until 6 May as they deal with the outbreak.
Soldiers have handed out six million masks in and around the capital region.
In Canada, cases were recorded at opposite ends of the country, in British Columbia and in Nova Scotia, while in Spain, a young man who had recently returned from Mexico was found to have the virus. He was said to be in a stable condition.
A number of countries in Asia, Latin America and Europe have begun screening airport passengers for symptoms, while Germany's biggest tour operator has suspended trips to Mexico.
Several countries have banned imports of raw pork and pork products from Mexico and parts of the US, although experts say there is no evidence to link exposure to pork with infection.
Shares in airlines have fallen sharply on fears about the economic impact of the outbreak.
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