About half of the raw chicken breasts in a nationwide sampling carried antibiotic-resistant "superbug" bacteria, a U.S. consumer group said on Thursday, calling for stricter limits on use of the medicines on livestock.

It could be more difficult to treat people if they became ill after eating chicken with the antibiotic-resistant bacteria, said Consumer Reports, which describes itself as the world's largest independent product-testing organization.

The group said it tested for six types of bacteria in 316 raw chicken breasts purchased from retailers nationwide during July. Almost all of the samples contained potentially harmful bacteria, it said.

Some 49.7 percent carried a bacterium resistant to three or more antibiotics, according to the group, and 11 percent had two types of bacteria resistant to multiple drugs. Resistance was most common for the antibiotics used for growth promotion and disease treatment of poultry.

Consumer Reports urged passage of a law to restrict eight classes of antibiotics for use only to treat humans and sick animals. The law would be more effective, it said, than the Food and Drug Administration's plan, announced last week, to phase down the non-medical use of antibiotics in livestock over three years.

In addition, it said the Agriculture Department should set levels for allowable salmonella and campylobacter bacteria in poultry and give its inspectors the power to prevent sale of poultry meat that contains salmonella bacteria that is resistant to multiple antibiotics.

Chicken is the most widely consumed meat in the United States. Americans are forecast to consume nearly 84 pounds per person in 2014, compared to 53 lbs of pounds of beef and 48 pounds of pork.

The broiler industry said it will cooperate with the FDA's planned phase-down of antibiotics although it says there is negligible risk from current use of the drugs.

Consumers should cook poultry to 165 degrees F (73.8C) to kill bacteria and take steps, such as using a separate cutting board for raw meat, to avoid cross-contamination of other foods, Consumer Reports said.