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

The worst-affected 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've 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.