OAK BROOK, Ill. — McDonald's said Monday it has stopped serving sliced tomatoes in its U.S. restaurants over concerns about salmonella food poisoning linked to some uncooked varieties.

The grocer Winn-Dixie Stores said it was also taking some tomatoes off its shelves. Other restaurant and supermarket chains reportedly halted some tomato sales as federal health officials worked to trace the source of the outbreak.

McDonald's Corp., the world's largest hamburger chain, stopped serving sliced tomatoes on its sandwiches as a precaution until the source of the salmonella is known, according to a statement Monday from spokeswoman Danya Proud.

McDonald's will continue serving grape tomatoes in its salads because no problems have been linked to that variety, Proud said.

Winn-Dixie Stores Inc. said it was voluntarily taking tomatoes involved in a Food and Drug Administration warning off its shelves. The Jacksonville, Fla.-based retailer operates 521 stores in Florida, Alabama, Louisiana, Georgia and Mississippi.

The source of the tomatoes responsible for the illnesses in at least 16 states has not been pinpointed. The Centers for Disease Control and Prevention has said at least 23 people have been hospitalized, and no deaths have been reported.

The FDA is investigating the source of the outbreak, FDA spokeswoman Kimberly Rawlings said in an e-mail. The FDA said Saturday the outbreak was linked to certain varieties of raw tomatoes including red plum, red Roma and red round.

In Pittsburgh, KDKA-TV reported that Giant Eagle supermarkets have removed several kinds of tomatoes from their shelves.

Fast-food chains Taco Bell Corp. and Chipotle Mexican Grill Inc. also have halted serving tomatoes, the Los Angeles Times reported. And supermarket chains Ralphs, Vons and Albertsons have stopped the selling red plum, red Roma or round red tomatoes cited by the FDA, the newspaper said.