ZURICH, Switzerland (AP) - The largest supermarket chain in Switzerland has blocked the sale of U.S. long grain rice after traces of an illegal genetically modified strain were found, a spokeswoman said Wednesday.

Retailer Migros detected small amounts of the strain LL 601 over the weekend in shipments of long grain rice coming from the United States, said spokeswoman Martina Bosshard. Swiss law prohibits the sale of genetically modified food without special authorization.

As a result, the company decided to hold back its entire stock of U.S. long grain rice and recall six products already sold to consumers, Bosshard said. Migros imports between 4,000 and 5,000 tons (4,400 and 5,500 U.S. tons) of the rice each year.

"We are now waiting for a definitive analysis before we proceed," Bosshard said, adding that the contaminated shipment had come from a single supplier, which she refused to identify.

The European Union, of which Switzerland is not a member, already imposed strict certification requirements on U.S. rice shipments in August because it had found traces of the illegal biotech strain.

On Tuesday, the European Commission said 33 of 162 samples of U.S. rice imports tested by European rice millers contained illegal genetically altered strains and had been recalled or withheld from the market.

Migros' rival in Swiss retailing, Coop, banned American long grain rice last week, blocking 10 silos containing about 800 tons (880 U.S. tons) as a precautionary measure, spokeswoman Liselotte Dolder said.

"We are blocking the rice until we have certainty and we are now working hard to test all our rice," Dolder told The Associated Press. She said the company was determined to prevent genetically modified goods from hitting its shelves.

The Swiss health ministry has yet to take any measures against U.S. long grain rice imports, but spokesman Martin Schrott said illegal LL 601 contamination would not be tolerated.

"We have taken note of Migros' and Coop's decisions, but we have not yet received any lab results from the relevant cantonal (state) authorities," Schrott said. If traces of the strain are found, he said the rice would be banned from sale.

Switzerland recommends importers obtain a certification that goods are free from biotech products when purchasing foreign suppliers. The EU requires such certification.

Concerns about the safety of biotech foods for consumers and the environment have led many Europeans to resist the introduction of such products, even if their use is widespread in the United States and other countries.

Governments in Germany and France, neighbors of Switzerland and two of Europe's largest economies, both have imposed national bans on products they deem unsafe.