The world's first known swine flu victim was a six-month-old baby girl in northern Mexico who had no known contact with pig farms, the head of a laboratory studying the virus said on Thursday.

"It's a six-month-old baby girl from San Luis Potosi who is alive" said Celia Alpuche of the Institute of Epidemiological Diagnosis and Reference (INDRE) in Mexico City.

The little girl first showed symptoms of the new strain of the influenza A(H1N1) virus on February 24, she said.

International attention has focused on two possible 'patient zeros', including a five-year-old boy who lived near a pig farm in eastern Mexico and a woman from Oaxaca, in the southeast, after the government first raised the A(H1N1) alert three months ago.

Both had contracted the virus, which has now killed more than 700 people worldwide, in April.

But studies carried out on a backlog of samples show that a first handful of recorded cases appeared in March in central and northern Mexico, before any showed up further south, said Ms Alpuche.

"We have other positive samples in March from Baja California (northwest), San Luis Potosi and Mexico City (centre)," Ms Alpuche said, referring to results discovered around one month ago.

"It's complicated to say where it originated but the earlier samples are not from rural areas, that's to say areas with farms (or) pigs," she added.

Mexico's swine flu death toll now stands at 138, with almost 14,800 recorded cases, and the country has recently seen an upsurge of cases in the impoverished southeast.

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