WASHINGTON (AP) -- The federal government's move to regulate e-cigarettes is a leap into the unknown. Most everyone agrees a ban on selling them to kids would be a step forward. But health and public policy experts can't say for certain whether the electronic devices are a good thing or a bad thing overall, whether they help smokers kick the habit or are a gateway to ordinary paper-and-tobacco cigarettes. The proposed rules, issued Thursday by the Food and Drug Administration, tread fairly lightly. They would ban sales to anyone under 18, add warning labels and require FDA approval for new products. Some public health experts say a measured approach is the right one. They think that the devices, which heat a nicotine solution to produce an odorless vapor without the smoke and tar of burning tobacco, can help smokers quit. "This could be the single biggest opportunity that's come along in a century to make the cigarette obsolete," said David Abrams, executive director of the Schroeder Institute for Tobacco Research and Policy Studies at the American Legacy Foundation. Still, some wonder whether e-cigarettes keep smokers addicted or hook new users and encourage them to move on to tobacco. And some warn that the FDA regulations could have unintended consequences. "If the regulations are too heavy-handed, they'll have the deadly effect of preventing smokers from quitting by switching to these dramatically less harmful alternatives," said Jeff Stier, senior fellow at the National Center for Public Policy Research, a conservative think tank in Washington. Scientists haven't finished much research on e-cigarettes, and the studies that have been done have been inconclusive. The government is pouring millions into research to supplement independent and company studies on the health risks of e-cigarettes and other tobacco products -- as well as who uses them and why. "There are far more questions than answers," acknowledged Mitch Zeller, director of the FDA's Center for Tobacco Products. But he said the proposed rules "would result in significant public health benefits, including through reducing sales to youth, helping to correct consumer misperceptions, preventing misleading health claims and preventing new products from entering the market without scientific review by FDA." The FDA has left the door open to further regulations, such as a ban on TV advertising and fruit- or candy-flavored e-cigarettes -- measures that some anti-smoking groups and members of Congress are demanding. "It is inexcusable that it has taken the FDA and the administration so long to act. This delay has had serious public health consequences as these unregulated tobacco products have been marketed using tactics and sweet flavors that appeal to kids," the Campaign for Tobacco-Free Kids said in a statement. The FDA said it wants more evidence before it rushes into more regulations. Any further rules "will have to be grounded in our growing body of knowledge and understanding about the use of e-cigarettes and their potential health risks or public health benefits," FDA Commissioner Dr. Margaret Hamburg said. Electronic cigarettes are becoming a big business. The industry started on the Internet and at shopping-mall kiosks and has rocketed from thousands of users in 2006 to several million worldwide, with a choice of more than 200 brands. Sales are estimated to have reached nearly $2 billion in 2013. Tobacco companies have noticed that e-smokes are eating into cigarette sales, and they have jumped into the business, too. Smokers like e-cigarettes because the nicotine-infused vapor looks like smoke but doesn't contain the thousands of chemicals, tar or odor of regular cigarettes. Some smokers use e-cigarettes as a way to quit smoking tobacco, or to cut down. "If the product as I use it now becomes illegal, I'm not sure what'll happen. I'll probably end up smoking again," said 38-year-old Jason Todrick of Huntington Beach, Calif., who kicked his more than 20-year smoking habit two years ago using an e-cigarette. In addition to mandating warning labels that say nicotine is an addictive chemical, the rules would require e-cigarette makers to disclose their products' ingredients. They would not be allowed to claim their products are safer than other tobacco products. In addition, they couldn't give out free samples or sell e-cigarettes in vending machines unless they are in a place open only to adults, such as a bar. The public and the industry will have 75 days to comment on the proposed rules. There is no timetable for when the FDA will issue its final rules. Many believe the process will wind up in court. "It seems to be a responsible approach ... and shows the FDA's commitment to look at particular e-cigarettes in a science-based way rather than just conjecture," said Jason Healy, president of Blu e-cigs, which is owned by the tobacco company Lorillard Inc. and is the largest player in the market. Blu accounts for almost half of e-cigarettes sold. Also on Thursday, the FDA proposed extending its authority to regulate cigars, hookahs, nicotine gels and pipe tobacco.