WASHINGTON - Greenpeace and organic farmers urged the Bush administration on Thursday to halt the planting of genetically modified crops as they inadvertently contaminate foods that claim to be free of genetically altered ingredients.

"You cannot build a wall high enough" to stop genetically modified organism crops from contaminating non-GMO crops, said Arran Stephens, president of Nature's Path Foods.

"We are calling for a moratorium on genetically engineered crops," Stephens said to reporters.

Worries about the purity of organic foods have grown as some American consumers choose so-called "natural foods" that declare they are free of genetically modified organisms.

Greenpeace's call for government action came after the Wall Street Journal reported on Thursday that its sampling of 20 vegetarian products showed that five contained "substantial" amounts of gene-spliced soybeans and about 11 others contained traces of bio-engineered soybeans.

The products were labeled as free of genetically modified foods. The Journal said it was not possible to determine if its sampling reflected an industrywide average.

In one vegetarian bacon product it tested, the Journal said, 40 percent of its soybean ingredients were from genetically modified plants.

The maker of that product, Yves Veggie Cuisine, told the newspaper it paid an extra $500,000 last year to suppliers for soybeans that were non-biotech varieties.

Charles Margulis, a genetic engineering specialist with Greenpeace, said: "The (Journal) report is not a surprise. Greenpeace has been warning this from the very start."

The risk of inadvertent contamination was clearly demonstrated last autumn, when huge amounts of the U.S. corn supply were mixed with StarLink corn, a variety not approved for human consumption. That triggered the recall of hundreds of U.S. taco chips, snack foods and other corn products.

Aventis SA, the maker of StarLink, last month revised up its estimate and said it now believes more than 430 million bushels of U.S. corn was contaminated. Meanwhile, U.S. farmers continue to embrace biotech soybeans as a way to improve their profits. Last week, the U.S. Agriculture Department said U.S. farmers would plant 63 percent of their soybean fields with gene-altered varieties this year, up 9 percent from last year. Soybean oil is widely used in processed foods.

Farmers have been rapidly switching to biotech soybeans because they produce superior yields and need fewer pesticides.

But David Vetter, an organic farmer in Nebraska who grows corn, soybeans and other crops, said the trend is placing huge financial burdens on his operation.

"We spend $1,500 in testing to sell $4,000 worth of corn," Vetter said. "We believe that no matter how much testing we do, we cannot guarantee anything to be free of GMO contamination."

Biotech plantings will account for 24 percent of American corn this year, down slightly from last year, according to the USDA. Industry experts say that reduction came about in part because of the StarLink episode.

The Food and Drug Administration, which regulates biotech foods, has refused to require labels identifying foods that contain genetically altered ingredients.

Instead, the FDA said in January it was developing guidelines for food makers who want to use terms such as "bioengineered" or "derived through biotechnology" on labels. The agency will not permit terms such as "GM free" or "modified" on food labels.

In addition to the guidelines, the FDA plans to begin requiring food companies to notify the federal government at least 120 days before selling a new biotech food on the market. Currently, only voluntary notification is required.

The FDA maintains that genetically altered foods are no different from conventional foods and need no special labels or procedures. Greenpeace and consumer groups have urged the government to require tests for health and environmental impacts.

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