WASHINGTON - Tobacco companies will be required to report the levels of dangerous chemicals found in cigarettes, chew, and other products under the latest rules designed to tighten regulation of the industry.

The preliminary guidance issued Friday by the Food and Drug Administration marks the first time tobacco makers will be required to report quantities of 20 chemicals associated with cancer, lung disease, and other health problems. The FDA will require companies to display the information in a consumer-friendly format by next April.

Constituents or by-products of tobacco products that are subject to the new rule include ammonia, carbon monoxide, and formaldehyde.

Regulators have identified more than 93 harmful or potentially harmful chemicals in tobacco products, though the agency is focusing on only 20 for the coming year. The agency will take comments on the guidance until June 4, before finalizing them.

A law enacted in 2009 gave the FDA authority to regulate a number of aspects of tobacco marketing and manufacturing, though the agency cannot ban nicotine. The same law lets the agency approve products that could be marketed as safer than what is currently for sale.

In separate guidance issued Friday, the FDA laid out the scientific studies it will require before any company can market a so-called modified-risk tobacco product. Companies must submit extensive testing data on health risks, user behavior, and consumer understanding of marketing materials for new products.

Industry experts expect it will take the FDA a year or more to review applications for modified-risk products.