"This could help currently addicted adult smokers who could switch to potentially less harmful products or, better yet, switch altogether," Mitch Zeller, director of the FDA's Center for Tobacco Products, said on a call with reporters. "Plus, it will keep future generations of kids who experiment from making the deadly progression from experimentation to addiction."

The Centers for Disease Control and Prevention estimates 37.8 million adults in the U.S. smoke cigarettes. An analysis published in the New England Journal of Medicine projects an additional 5 million adult smokers could quit within one year of introducing nicotine limits, and that such a policy could help lower the smoking rate to as low as 1.4 percent from the current 15.5 percent.

The agency is publishing an advanced notice of proposed rule-making, a first step in the process to require the level of nicotine in cigarettes be at minimally or nonaddictive levels.

The Food and Drug Administration is taking the first steps toward implementing a rule about the maximum level of nicotine in combustible cigarettes, after an announcement of its plans last summer that rocked stocks of tobacco companies.

The document lays out questions the FDA is seeking input on, such as what level of nicotine it should limit cigarettes to and how to measure it. Another area it's seeking guidance on is how it would implement a standard nicotine level, such as introducing a new, lower level at once, or gradually reducing it over time.

The FDA also wants to know what the unintended consequences of lowering levels of nicotine could be, such as people inhaling deeper or smoking more cigarettes to compensate for the difference.

Another consideration is the feasibility of techniques to lower nicotine in cigarettes. Possible techniques include tobacco blending, cross-breeding plants, genetic engineering and chemical extraction, the agency said in its notice.

"As this process gets underway, we look forward to working with FDA on its science-based review of nicotine levels in cigarettes and to build on the opportunity of establishing a regulatory framework that is based on tobacco harm reduction and recognizes the continuum of risk," said James Figlar, executive vice president of RAI Services, an indirect subsidiary of Reynolds American, and executive vice president of research and development for R.J. Reynolds Tobacco.

The tobacco industry has said it welcomes Gottlieb's approach to regulation, which embraces the idea that tobacco products exist on a continuum of risk. The concept means that some nicotine products are safer than others.

Smoke particles in combustible cigarettes are the most harmful. Igniting tobacco causes it to undergo a chemical process that emits toxins.

Elsewhere on the spectrum include products like e-cigarettes, which deliver nicotine-laced vapor. Cigarette makers have invested in these, as well as heat-not-burn products, which warm tobacco sticks to a temperature that's high enough to release an aerosol but not enough to cause combustion.

A Philip Morris International spokesman said the company is still in the process of reviewing the document but "has previously expressed support for Commissioner Gottlieb's comprehensive tobacco nicotine and regulation plan, which encourages the development of innovative new tobacco products that may be less harmful than cigarettes."

PMI is seeking approval from the FDA to sell its heat-not-burn device, iQOS, in the U.S. The application is still pending, and if approved, Altria would market the product in the U.S. In January, an independent advisory committee recommended the FDA not approve PMI's application to market iQOS as safer than cigarettes, though the FDA will ultimately make the final decision.

"As FDA has acknowledged, any proposed nicotine standard would need to be part of a comprehensive package which also includes steps to 'encourage the innovation of less harmful products,'" said Altria Executive Vice President and General Counsel Murray Garnick. Altria has already been preparing for any reasonable potential standard, and we plan to participate in every step of this process."

Shares of Altria, which sells Marlboro in the U.S., closed unchanged, while shares of PMI, which markets the brand outside of the U.S., fell 0.3 percent. British American Tobacco's stock dipped 0.5 percent. The company acquired Reynolds last year.

Wells Fargo analyst Bonnie Herzog called the release "more of a headline risk than a debate changer," in a note to clients. The actual process itself could take awhile, and it could be years before an ultimate rule is issued and implemented, she wrote.

"Uncertainty remains as to what the FDA may ultimately propose [and] how the industry may adapt," Herzog wrote. "We continue to believe in an environment where nicotine levels in [cigarettes] are reduced, conversion to [reduced-risk products] will accelerate and [Altria] (and [Philip Morris]) are strongly positioned with iQOS."

The next steps include soliciting public input before moving toward a potential draft rule.

The FDA also said it will soon file two additional advance notices of proposed rule-making seeking input on the role flavors, including menthol, play in tobacco use as well as regulation of premium cigars.