Some chemically enhanced caramel food colorings used in widely consumed cola drinks could cause cancer and should be banned, a U.S. consumer advocacy group urged the Food and Drug Administration.

Pure caramel is made from melted sugar; but two other versions approved to color food products include the chemical ammonia and produce compounds shown to cause various cancers in studies of animals, the Center for Science in the Public Interest (CSPI) said in a statement.

The group is petitioning the FDA to ban the ammonia-containing caramels, which are also used in other dark-colored soft drinks.

Coca-Cola Co, the world's top soft drink maker, said the caramel it uses does not cause cancer.

It said its cola only contains one of the compounds cited by CSPI, and that the compound -- formed in the "browning reaction" while cooking -- is found in trace amounts in a variety of food and drinks.

PepsiCo Inc referred a call to the American Beverage Association, while Dr Pepper Snapple Group Inc was not immediately available to comment.

Obesity is still a greater health threat from soda, the CSPI said. But the chemical reaction between sugar and ammonia can form carcinogens and "still may be causing thousands of cancers in the U.S. population," the group said, citing animal studies conducted by government researchers at the National Institutes of Health's National Toxicology Program.

"The American public should not be exposed to any cancer risk whatsoever as a result of consuming such chemicals, especially when they serve a non-essential, cosmetic purpose," several of the NIH scientists wrote in a letter to the FDA.

CSPI executive director Michael Jacobson, in a statement, also said use of the words "caramel coloring" on food labels was misleading and should not be allowed.

The American Beverage Association fired back, calling CSPI's claim a "scare tactic" and said there was no evidence that the compounds found in caramel coloring cause cancer in humans.