KUALA LUMPUR - Filipino farm workers living by a field of gene-modified maize showed signs of exposure to the plant's anti-pest toxin three months after the pollen season, Norwegian scientist Terje Traavik said on Monday.

Blood samples from 39 people in a farm community in Mindanao carried increased levels of three different target antibodies, evidence of an immune reaction to the Bt toxin built in to combat pests, he added.

"We are absolutely sure it's a reaction to being exposed to the Bt maize," Mr. Traavik told Reuters on Monday at the start of international talks on trade in genetically modified (GM) crops.

The timing of the immune response, which was not equivalent to conventional hay fever, coincided with maize flowering in June.

If more tests were to confirm his findings, they would fuel anti-GM campaigner arguments that extra caution is needed before wide-scale cultivation of modified crops such as maize, canola and cotton goes any further.

Biotech companies say their GM products are as safe as "substantially equivalent" natural varieties and need no additional safety tests or measures.

Mr. Traavik said the maize variety involved, sold as Dekalb 818 YG, came from US crop company Monsanto.

But Willy De Greef, a biotech law consultant formerly employed by Swiss agrochemicals company Syngenta, expressed surprise at Traavik's findings, saying research showed Bt maize pollen did not carry the toxin so no reaction should occur.

"One would want a scientific panel to look at Mr. Traavik's results," he said on the margins of the Malaysia meeting.

In Davao City, Philippines, activists said Traavik's findings were unexpected. "It's something new to us here in Mindanao," said Dr. Hospicio Cunanan, head of Konsumo Dabaw, an advocacy group on environmental and consumer rights. Konsumo Davao is active in the local fight against Bacillus thurginesis (Bt) corn. "There was monitoring conducted on farmers involved in Bt corn and its adverse impact on the health; that's what Sister Susan Bolanio tried to prove," said Socorro Requiza, a director of the group.

Sister Bolanio, justice and peace desk coordinator of the diocese of Marbel (Koronadal City in South Cotabato), has been at the forefront of the campaign against Bt-corn.

Genetically modified corn has been tested by Monsanto since late '90s in farms in Southern and Northern Mindanao. The planting of Bt-corn has been endorsed by the Department of Agriculture (DA).

In Manila, DA belied Mr. Traavik's findings. Artemio M. Salazar, program director of DA's corn program, said, "What [Mr. Traavik] is saying is impossible. Humans cannot just develop immunity from certain antibodies by simple exposure to GM corn plants either by planting it or eating it."

He also said the commercial propagation of Monsanto's Bt corn followed procedures that would ensure safety.

Monsanto earlier claimed the farm area planted to Bt corn has expanded to 12,000 hectares since commercial propagation started last year.

Mr. Salazar said DA could probe the matter if it was brought to their attention.