The Food and Drug Administration issued warning letters to the makers of Winston, Natural Spirit and Nat Sherman cigarettes over their "additive-free" and "natural" label claims.

The agency issued the warnings to ITG Brands LLC, Santa Fe Natural Tobacco Company Inc. and Sherman's 1400 Broadway N.Y.C. Ltd. The issue over the claims is that they may lead consumers to believe the products pose a lower risk. That claim has to be scientifically proven.

In a statement, the FDA said it has determined that the products under the warning letter need what is called a "modified risk tobacco product order" before they can be marketed in that way. It has not issued any orders for modified-risk products to the market and this is the first time it is using its authority to take action against "natural" or "additive-free" claims.

The companies have 15 days to respond with a plan or dispute the warnings.

There was no immediate response from the companies.

Imperial Tobacco Group Plc. owns ITG Brands, which also makes Kool cigarettes and USA Gold. Reynolds American Inc. owns Santa Fe Natural Tobacco.

The warning comes several days after a large group of anti-tobacco organizations sent the FDA a letter urging the agency to enforce regulations against Santa Fe Natural Tobacco over marketing claims. That letter, sent on Monday, was signed by 29 groups including the American Heart Association, American Legacy Foundation and Campaign for Tobacco-Free Kids.

The anti-tobacco group's letter alleged that Natural American Spirit's advertising in magazines such as Sports Illustrated and Vanity Fair violated the Smoking Prevention and Tobacco Control Act.

"The potential for irreparable damage to public health from the marketing of tobacco products with modified risk claims is well illustrated by the industry's years of deceptive advertising of 'light' and 'low-tar' cigarettes," the letter stated.