The U.S. Food and Drug Administration sent warnings to five makers of electronic cigarettes for marketing them illegally as stop-smoking aids and said on Thursday it intends to regulate the products as drugs.

The move is the latest attempt by the FDA to assert its jurisdiction over electronic or e-cigarettes, battery-powered devices that allow users to inhale a vaporized liquid nicotine solution instead of tobacco smoke.

Michael Levy of the FDA's Center for Drug Evaluation and Research told a news briefing the warnings were for violations of the Federal Food, Drug and Cosmetic Act, including making unsubstantiated claims and poor manufacturing practices.

Under the act, a company cannot claim that its drug can treat a disease, such as nicotine addiction, unless the drug's safety and effectiveness have been proven.

Yet all five companies claim that their products help users quit smoking.

We are interested in finding out whether e-cigarettes can be proven safe and effective. That is why we sent out the letter to the Electronic Cigarette Association, he said.

Companies named in the letters include E-Cigarette Direct LLC; Ruyan America Inc; Gamucci America, also known as Smokey Bayou Inc; E-Cig Technology Inc and Johnson's Creek Enterprises LLC.

The action only concerns the five companies that got warning letters, but Levy said FDA will continue to evaluate the marketers of e-cigarettes on a case-by-case basis.

We have not made a decision to remove all e-cigarettes from the market, Levy said.

In addition to the stop smoking claims, some of the companies were targeted because they sell medications in liquid form to be used in cartridges that become vaporized and can be inhaled by users.

E-Cig Technology, for example, markets the erectile dysfunction drug tadalafil, sold by Eli Lilly as Cialis, and a liquid version of the weight-loss drug rimonabant or Compal, a Sanofi-Aventis drug that never won U.S. marketing approval and was pulled from the market in Europe because of safety concerns.

These liquid medications are designed to refill cartridges used in e-cigarettes so that the drugs can be vaporized and inhaled, the FDA said in a statement.

The warning letters to the companies say they have 15 days to respond. The FDA will wait to act depending on what the companies say, Levy said.

Matthew Myers, president of the Campaign for Tobacco-Free Kids, applauded the U.S. push to regulate e-cigarettes.

To protect public health, the FDA must hold electronic cigarettes to the same safety, efficacy and manufacturing standards as other drugs, including nicotine replacement products, Myers said in a statement.

A U.S. judge in January granted a preliminary injunction barring the Obama administration from trying to regulate electronic cigarettes and prevent them from being imported into the United States.

Levy declined to comment on that case.