The U.S. Food and Drug Administration allowed potentially harmful antibiotics to be used in feed for livestock - despite an internal review that found some of the drugs posed a "high risk" of exposing humans to antibiotic-resistant bacteria through the food supply, according to a study released Monday by an environmental advocacy group.

The Natural Resources Defense Council, a non-governmental group, found that the FDA allowed penicillin and tetracycline antibiotic feed additives, including 18 rated as "high risk," to remain on the market as additives in farm animal feed and water, Reuters reported.

"The evidence is clear," Carmen Cordova, NRDC microbiologist and lead author of the new analysis, said in a statement. "FDA continues to knowingly allow the use of drugs in animal feed likely pose 'high risk' to human health. That's a breach of their responsibility and the public trust."

The NRDC report claims that none of the 30 antibiotics, which were scientifically reviewed by the FDA between 2001 and 2010 and "raised significantly red flags," would likely be approved as new additives for livestock use if submitted under current FDA guidelines, because drugmakers have not submitted sufficient information to establish their safety. NRDC also contends that 18 of the 30 drugs were deemed to pose a "high risk" of exposing humans to antibiotic resistant bacteria through the food supply and of adversely affecting human health.

"This discovery is disturbing but not surprising given FDA's poor track record on dealing with this issue. It's just more overwhelming evidence that FDA - in the face of a mounting antibiotic resistance health crisis - is turning a blind eye to industry's misuse of these miracle drugs," Cordova said of the group's findings.

In their report, the environmental group said the findings extend beyond the 30 antibiotic feed additives reviewed for the study and could lead to bacterial resistance to other antibiotics, reducing the effectiveness of a range of other medically important antibiotics that are solely used to treat people.

NRDC attorney Avinash Kar, told The Washington Post that the group's analysis underscores a larger pattern of FDA inaction "in combating the overuse of antibiotics in animals," which is contributing to the rise of hard-to-treat infections.

"They are not meeting their responsibility to protect public health," Kar said. "They need to be doing better."