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MEXICO CITY, June 12 (Reuters) - U.S. buyers spooked by a salmonella outbreak are blocking shipments of Mexican tomatoes at the U.S. border as reports come in that the bacteria has poisoned over 200 people in 23 states, Mexico's agriculture minister said on Thursday.

"I've had phone calls from producers saying their tomatoes are being blocked, not all varieties but some varieties," Alberto Cardenas said, stressing that U.S. officials had found no evidence so far that Mexican tomatoes were unsafe to eat.

Twenty-five people have been hospitalized as a result of the U.S. outbreak, which is being linked to raw plum, Roma and round tomatoes. Investigators at the U.S. Food and Drug Administration, or FDA, have not ruled out Mexico as the source of the infection.

"There is uncertainty, and once a producer sends a shipment, it is complicated to get it back so they are holding off to see what the FDA says," said Manuel Tarriba, head of a vegetable exporters association.

Many growers had to throw out their fresh crops after buyers canceled orders, he said.

Farmers and the government say Mexico's industry follows strict sanitary standards and producers were being singled out unfairly amid the panic.

"Our tomatoes have been eaten in the United States for years and there has never been a problem of any kind. Here we eat Mexican tomatoes all year round and there has never been an incident," Cardenas said.

Some 84 percent of the tomatoes imported by the United States come from Mexico, around 700,000 tonnes a year in a business worth $900 million, according to Tarriba's organization.

Exports of Mexican agricultural products soared after the United States, Canada and Mexico lifted trade barriers under the 1994 North American Free Trade Agreement.

But Mexican farmers say the advantages of free trade can be easily canceled out by a sanitary scare. (Reporting by Mica Rosenberg and Adriana Barrera; Editing by Marguerita Choy)

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