CHICAGO - McDonald's, Wal-Mart, and other U.S. chains have halted sales of some raw tomatoes as federal health officials work to trace the source of a multistate salmonella food-poisoning outbreak.

Burger King, Outback Steakhouse and Taco Bell were among other restaurants voluntarily withdrawing tomatoes from their menus, following federal recommendations that consumers avoid red plum, red Roma or round red tomatoes unless they were grown in certain states and countries.

McDonald's Corp., the world's largest hamburger chain, has stopped serving sliced tomatoes on its sandwiches as a precaution until the source of the bacterial infection is known, according to a statement yesterday from spokeswoman Danya Proud. McDonald's will continue serving grape tomatoes in its salads because no probmems have been linked to that variety, she said.

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.

Pennsylvania, New Jersey and Delaware did not report any cases.

The Food and Drug Administration warned consumers in New Mexico and Texas as early as June 3 about the outbreak. The agency expanded its warning during the weekend and chains began voluntarily removing many red plum, red Roma or round red tomatoes from their shelves in response.

Other companies or restaurants that said they had stopped selling or serving certain raw tomatoes and, in some cases, also discarded salsa and some other prepared foods, included Subway, Burger King, Outback Steakhouse, Carrabba's, Bonefish Grill, Red Lobster, Olive Garden, Chipotle Mexican Grill, Taco Bell, Wal-Mart, Winn-Dixie Stores and Publix Super Markets.

The FDA is investigating the source of the outbreak, agency spokeswoman Kimberly Rawlings said.

Cherry tomatoes, grape tomatoes, tomatoes sold with the vine still attached and homegrown tomatoes are likely not the source of the outbreak, federal officials said.