MEXICO CITY -- Mexico is trying to limit the importation of genetically modified corn from the United States after a NAFTA watchdog group recommended better regulation of the crop, something U.S. officials have said is unnecessary.

This country imports about 5.6 million tons of American corn a year. Between 30 percent and 50 percent of that corn is genetically modified.

A study released Monday by the trilateral Commission for Environmental Cooperation said biotech corn is not likely to displace Mexico's native species, which gave rise to modern corn.

But it said no one knows for sure how much genetically modified corn has already been mistakenly planted in Mexican fields and recommended that steps be taken to slow the amount of biotech corn pouring across the border.

"Trans-genes are not more risky than other varieties of corn. They are not likely to reduce the genetic diversity of maize," said Chantal Line Carpentier, the report's coordinator. "But if it keeps coming in and keeps getting planted, you increase your chances of risk."

The report suggested Mexico mill biotech corn grains as soon as they cross the border to ensure they are used only in animal feed and not planted.

About 45 percent of the corn produced in the United States is genetically modified. Most of that is altered to produce a naturally occurring toxin known as Bacillus Thuringiensis, or Bt, to ward off pests.

The report also recommended U.S. exporters label all corn that has been genetically modified and that Mexico better educate its small farmers about the dangers of planting biotech corn.

The study was done at the request of 21 Indian communities in Oaxaca, which in April 2002 had asked for an informative analysis.

The United States has called Mexico's effort to limit imports unnecessary, dismissing the report that spawned it as "fundamentally flawed and unscientific."

"Implementing many of the report's recommendations would cause economic harm to farmers," said a joint statement by the office of U.S. Trade Representative Robert Zoellick and the Environmental Protection Agency.

In the past, trade groups have complained similar recommendations would add unfair costs for exporters.

"While the report's authors recommend that biotech maize be treated differently from other maize hybrids, science tells us the opposite," the statement said. "In fact, the findings of this report echo the prevailing science, supporting our view that biotech maize will have no greater or lesser effect on maize genetic diversity than other modern maize hybrids."

Don Doering, of the 16-member advisory group that authored the report, said that while the risk to biological diversity from the modified corn is minimal, Mexico needs to take steps to reassure thousands of subsistence corn farmers who are terrified that genetically modified corn stalks might consume their fields.

"A big piece of this is, you have a developing country that's upset and uninformed and unprotected," he said. "Much of the Mexican population wants and needs to feel safer about all of this."

Farmers in Mexico first bred modern corn some 6,000 to 8,000 years ago. The country is home to at least 59 species of maize, from the protein-rich variety used to make tortilla chips to a softer grain mashed for use in tamales.