'International concern' about Swin Flu

In the US, seven people in California, two people in Texas, and two people in Kansas have been infected with the new strain.

In New York, city health commissioner Dr Thomas Frieden said preliminary tests conducted on the ailing students showed they were possible cases of swine flu.

Further tests will clarify if it was the same strain that was detected in the other three states.

Following a meeting of its emergency committee on Saturday, the WHO said the virus had the potential to become a pandemic but it was too early to say whether that would happen.

WHO Director General Margaret Chan said recent events constituted "a public health emergency of international concern" and that countries needed to co-operate in heightening surveillance.

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.

Officials said most of those killed so far in Mexico were young adults - rather than more vulnerable children and the elderly.

There is currently no vaccine for the new strain but severe cases can be treated with antiviral medication.

It is unclear how effective currently available flu vaccines would be at offering protection against the new strain, as it is genetically distinct from other flu strains.