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

The group is petitioning the Food and Drug Administration (FDA) to ban the ammonia-containing caramels, which are also used in other dark-coloured soft drinks.

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

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

"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 National Institutes of Health scientists wrote to the FDA.