WASHINGTON - Federal health officials are cracking down on underage use of a popular e-cigarette brand following months of complaints from parents, politicians and school administrators.

The Food and Drug Administration said Tuesday it had issued warnings to 40 retail and online stores as part of a nationwide operation against illegal sales of Juul to children. 

FDA regulators also are asking manufacturer Juul Labs to turn over documents about the design, marketing and ingredients of its devices. The rare request particularly focuses on whether certain product features are directly appealing to young people.

Like other e-cigarettes, Juul is an electronic device that vaporizes liquid nicotine into an inhalable vapor. Thanks in part to its resemblance to a small computer flash drive, Juul has become popular with some teenagers as a discreet way to vape at school and in public.

"In some cases, our kids are trying these products and liking them without even knowing they contain nicotine," FDA Commissioner Scott Gottlieb said in a statement. He said the agency plans additional action in coming weeks.

"This isn't the only product that we're looking at, and this isn't the only action we're going to be taking to target youth access to tobacco products, and e-cigarettes, in particular," he said.

Health advocates have worried about the popularity of vaping products among kids and the potential impact on adult smoking rates in the future. A recent government-commissioned report found "substantial evidence" that young people who use e-cigarettes are more likely to try cigarettes; while a study published in Tobacco Control this month found that 63 percent of Juul users didn't realize the products always contains nicotine.

The Associated Press contributed to this report.