US government and academic representatives were among those at the meeting of the AOAC, a group of scientists that works on testing issues with US Department of Agriculture inspectors and others in the government.

Smith, who is on an international tour calling for extensive testing of all biotech foods, said independent evaluations of genetically modified food crops has shown numerous indications of health problems. He cited stomach lesions in rats, false pregnancies in cows, excessive cell growth and damage to animal immune systems.

Smith said the US government has approved biotech products based on company research. He said scientists have been fired, and otherwise penalized for raising red flags about the technology.

"We don't know what will happen," Smith said. "We need long-term testing."

Proponents of biotech downplayed safety issues, saying gene modification and transfer occurs naturally in nature and is not dangerous. They said government oversight is adequate and they outlined a range of benefits farmers enjoy from biotech crops such as increased production.

Risks of toxicity and increased allergens that may be present in biotech foods may also exist in conventional foods, said Gregory Conko, director of food safety policy at the Competitive Enterprise Institute, an organization aimed at advancing free enterprise and limited government

"A lot of these risks are in fact present... but they are also present in conventional breeding technology," Conko said. "There are pretty good ways of determining if a plant is safe enough to be put on the market."

The debate in Kansas City is part of the global sparring underway over the issue of genetically modified crops, which are pervasive in America but largely shunned in Europe and many countries.

Earlier this week Monsanto Co., the leading US developer of biotech soybeans and corn, came under attack in European press reports over internal company research that found possibly detrimental health changes in rats fed Monsanto's biotech corn.

Monsanto said the results of the study, which found that rats fed the biotech corn had smaller kidneys and blood composition different from rats not fed the biotech corn, could be explained by statistical variations and was not evidence of any negative health impacts associated with its corn.