WASHINGTON — (CNN) In a crackdown on the e-cigarette industry, the Food and Drug Administration has issued warning letters to dozens of retailers that violated new rules by selling e-cigarettes, hookah tobacco and other tobacco products to minors.

The retailers included some specialty stores, online establishments and major national chains, including 7-Eleven, Chevron and Walgreens, the administration announced Thursday.

“We’re helping protect the health of America’s youth by enforcing restrictions that make it illegal to sell tobacco products to minors — including e-cigarettes, e-liquids and cigars. Retailers play a vital role in keeping harmful and addictive tobacco products out of the hands of children and we urge them to take that responsibility seriously,” said Mitch Zeller, director of the FDA’s Center for Tobacco Products, in a written statement.

The warning letters went out about one month after the FDA banned the sale of all tobacco products, including e-cigarettes, to anyone under 18. The administration also required retailers to check the photo IDs of all tobacco customers under 27.

The new regulations were finalized in May and effective August 8.

Once the regulations were established, the FDA conducted compliance checks at various retailers across the country.

Officials discovered 55 retailers where minors were able to purchase some tobacco products, such as e-cigarettes, cigars, hookah and pipes.

Some of the products included “youth-appealing” flavors, the administration said, such as e-cigarette liquids flavored “Cap-N-Crunch Berries” and “Strawberry Cake Pop.”

E-cigarettes, or electronic cigarettes, contain a liquid composed of flavorings, propylene, glycol, glycerin and nicotine.

The liquid heats up inside of the device until it vaporizes, creating a vapor that the user can inhale or “smoke.”

Some of the retailers that were issued warnings are in Washington, South Carolina, Maryland, Arizona, Iowa, Arkansas, Maine, Missouri, Wisconsin, Mississippi, Rhode Island and Connecticut, according to a list compiled by the FDA (PDF).

When violations occur, the agency generally issues warning letters first before it may pursue enforcement actions, such as fines or no-tobacco-sale orders.

Prior to the new regulations, the FDA did not prohibit the sale of such tobacco products to minors — and while smoking traditional cigarettes has been on the decline among teens in the United States, their use of other tobacco products, such as e-cigarettes, has continued to climb.

A report released by the Centers for Disease Control and Prevention in April found that the number of high school students who said they use e-cigarettes jumped from 1.5% in 2011 to 16% in 2015.

The number who said that they smoke hookah rose from 4.1% to 7.2%.

The World Health Organization calls tobacco use one of the biggest public health threats the world has ever faced, as it kills about 6 million people each year.