The Food and Drug Administration is asking the company behind leading e-cigarette brand Juul for information on how its products appeal to kids and teens, opening the door to possible enforcement actions.

The agency also put retailers on notice that it's performing undercover work to see whether they're illegally selling to minors. So far, it's issued 40 warning letters.

The efforts come as the Juul craze spreads across the nation. The e-cigarette has become popular among young people, prompting public health officials and lawmakers to urge the FDA to take some sort of action.

Critics argue flavors like "creme brulee" attract kids and mask the fact that one pod contains as much nicotine as a pack of cigarettes. The device, which is as small as a flash drive, is easy to hide from parents. In a statement, Gottlieb acknowledged these characteristics may make e-cigarettes attractive to youth.

In a rare move, the FDA is issuing a 904(b) letter, which refers to the section of the Family Smoking Prevention and Tobacco Control Act. It's the first time in three years that the FDA has sent such a letter.

In this one, the agency is asking Juul for a slew of company materials, including marketing documents, research on whether certain products' design features, ingredients or specifications appeal to different age groups.

The FDA will review the information and if there are reasons for concern, it could take enforcement actions. It also plans to issue additional letters to other manufacturers. If they don't comply with requests, they will be violating the law and subject to enforcement.

"We don't yet fully understand why these products are so popular among youth," FDA Commissioner Scott Gottlieb said in a statement. "But it's imperative that we figure it out, and fast. These documents may help us get there."

Juul said in a statement that it agrees with the FDA that "illegal sales of our product to minors is unacceptable" and that it has programs in place to prevent and "identify and act upon these violations at retail and online marketplaces, and will announce additional measures in the coming days.

"We are working with the FDA, lawmakers, parents and community leaders to combat underage use, and will continue working with all interested parties to keep our product away from youth," it added.

Gottlieb also said Tuesday it has issued 40 warning letters for retailers illegally selling Juul products to minors since the beginning of March. The agency started conducting an undercover blitz nationwide on April 6 to crack down on underage sales both in-stores and online. It will continue to the end of the month.

eBay removed several listings for Juul products and voluntarily implemented new measures to prevent others like them from appearing on the website after the FDA contacted the company, Gottlieb said.

"Let me be clear to retailers. This blitz, and resulting actions, should serve as notice that we will not tolerate the sale of any tobacco products to youth," Gottlieb said in a statement.

In response to reports of adolescents using its products, Juul has invested in education and prevention efforts such as "secret shoppers" who test to make sure retailers are not selling to minors, Juul Labs' chief administration officer Ashley Gould told CNBC in December.

Anti-tobacco groups Campaign for Tobacco-Free Kids and Truth Initiative applauded the FDA's announcement but said the moves aren't enough. They called on the agency to pull fruity flavors off the market and require e-cigarettes to undergo regulatory review.

Last summer, the FDA delayed implementing such a rule. The two groups and a handful of others sued the agency over that decision last month.

The FDA is planning on announcing more enforcement actions focused on companies the agency thinks are marketing products in ways that are misleading to kids, Gottlieb said.

"America's youth should not be using tobacco products," Health and Human ServicesSecretary Alex Azar said in a statement. "We at HHS are proud of the work Commissioner Gottlieb and the FDA are doing to crack down on the sale and marketing of e-cigarettes to minors."

Gottlieb announced a sweeping overhaul of tobacco regulation last summer, which includes a plan to lower the amount of nicotine in conventional cigarettes to minimally or non-addictive levels. It could encourage adult smokers to switch to nicotine alternatives, like patches, gums, lozenges — and e-cigarettes.

The plan is based on the idea that tobacco products exist on a continuum of risk, with conventional cigarettes being the most deadly. Critics have scrutinized this component, arguing the FDA doesn't know how safe e-cigarettes truly are.

They also point to Juul and other e-cigarettes' popularity spreading among teens, possibly hooking another generation to nicotine after years of cutting smoking rates.

This leaves Gottlieb a balancing act. On one hand, he sees these alternatives as possibly less harmful alternatives for adult smokers who still crave nicotine. On the other, he said he recognizes the risk in addicting minors.

"Make no mistake. We see the possibility for (electronic nicotine delivery systems) products like e-cigarettes and other novel forms of nicotine-delivery to provide a potentially less harmful alternative for currently addicted individual adult smokers who still want to get access to satisfying levels of nicotine without many of the harmful effects that come with the combustion of tobacco," Gottlieb said in a statement.

"But we've got to step in to protect our kids."