The U.S. Food and Drug Administration warned five electronic cigarette makers Thursday that they were violating federal law.

The agency said the products, which use a device to turn nicotine liquid into a vapor mist, are drugs that require pre-market approval similar to other nicotine replacement products. The FDA recently started regulating traditional cigarettes under a separate tobacco law.

The FDA said the e-cigarette companies are marketing their products as tools to help people quit using cigarettes. The agency suggested the companies would need to conduct clinical studies showing the products are an effective treatment for nicotine addiction.

The companies receiving warning letters are E-CigaretteDirect LLC of Parker, Colo.; Minneapolis's Ruyan America Inc.; Gamucci America, also known as Smokey Bayou Inc., of Jacksonville, Fla.; E-Cig Technology Inc. of Las Vegas and Johnson Creek Enterprises LLC of Johnson, Wis.

The Electronic Cigarette Association, which represents e-cigarette companies, didn't immediately return a call for comment. The five companies represent a small portion of the estimated 300 firms that make or distribute electronic cigarettes.

The FDA said it also cited Johnson Creek Enterprises, which markets Smoke Juice, a liquid solution used to refill depleted cartridges in e-cigarettes, for deficiencies in its manufacturing processes and cited E-Cig Technology for using tadalafil, an erectile dysfunction drug, and rimonabant, a weight loss drug that's not approved for use in the U.S., in some of its products.

The companies have 15 days to respond to the FDA.

Michael Levy, director of labeling compliance for FDA's drug division, said the agency would wait to see what the company responses were before deciding what to do next.

The agency could act to remove the products from the market but it would likely involve a court fight.

FDA is already involved in a separate lawsuit with other e-cigarette companies that involves the agency's jurisdiction over the products. In 2008 the FDA started detaining some shipments from China on the grounds that they were unapproved drug devices.

Levy said the FDA cannot discuss that lawsuit.

-By Jennifer Corbett Dooren, Dow Jones Newswires; 202-862-9294; jennifer.corbett@dowjones.com