Washington - US President Barack Obama is asking Congress for an added 1.5 billion dollars to fight the swine flu outbreak that has spread to five US states, his spokesman said Tuesday. The money will be used to "enhance the nation's capability to stop the spread," including stockpiling anti-viral drugs and develop and "ramp up production" of a vaccine, said Robert Gibbs.

Gibbs dismissed the suggestion that the money request indicated a new level of concern by the White House. "In our opinion, this is about prudent planning," he said.

The number of US human swine influenza cases in the United States increased to 64 in five states, the US Centres for Disease Control and Prevention (CDC) said Tuesday.

Some of the money will also be used in assisting international outbreaks of the swine flu, Gibbs said. Twelve million doses of anti- viral drugs were being distributed to the states affected, which include New York and California.

The worst-affected state was New York City, with 45 cases, while California reported 10 cases. There were 6 cases in Texas, 2 in Kansas and 1 in Ohio.

Those who have fallen ill in the US range in age from 7-54 years, with a median age of 16 years. There have been no deaths in the US.

Mexico is experiencing the worst outbreak, with 152 deaths from an influenza-type illness, but only a handful have been attributed as of yet to swine flu. Small numbers of non-fatal cases have also been confirmed in a number of other countries.

In response to the intensifying outbreak, the World Health Organization has raised the worldwide pandemic alert level to Phase 4, which means they have determined that the virus is spread through sustained person-to-person contact.

The CDC and State Department issued an advisory late Monday urging against non-essential travel to Mexico, in effect through July.

Health experts were struggling to understand swine flu, which has genetic elements that come from three species - pigs, birds and humans - and has never been seen before.

No one has, so far, been seriously ill or died in the US, and the infections have been self-resolving, with the disease running its course and the patients recovering. (dpa)