They analysed samples from people infected with the new H1N1 swine flu virus, which has been confirmed in more than 19,000 people in 64 countries, killing about 120. US health officials say this number reflects only a fraction of the true number of cases.

'This virus most likely evolved from recent swine viruses,' Gerardo Nava of the National Autonomous University of Mexico and colleagues wrote in their report, published in the online journal Eurosurveillance.

'These findings indicate that domestic pigs in North America may have a central role in the generation and maintenance of this virus.' The global pork industry has rushed to defend pig products, saying pig meat is no danger to people.

But health experts have also noted there is very little surveillance done to track influenza among pigs - even though the virus is very common in the animals and just as transmissible as it is among people.

Flu viruses have also been shown to pass from pigs to people and from people to pigs.

'These observations also reiterate the potential risk of pig populations as the source of the next influenza virus pandemic,' Dr Nava and colleagues wrote.

'Although the role of swine as 'mixing vessels' for influenza A(H1N1) viruses was established more than a decade ago, it appears that the policy makers and scientific community have underestimated it.' -- REUTERS