WASHINGTON — The Food and Drug Administration on Tuesday announced a nationwide undercover “blitz” to crack down on the sale of e-cigarettes – particularly the hugely popular Juul products – to children and teenagers by regular and online retailers.

The effort started April 6 and will continue to the end of the month. The agency said it has uncovered dozens of violations of the law and issued 40 warning letters related to Juul e-cigarettes.

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

Juul e-cigarettes resemble a USB flash drive but contain high levels of nicotine. They come in such flavors as mango, creme brulee and cool mint and their emissions can be virtually invisible, making it difficult for teachers to spot use of the product.

The FDA has asked Juul Labs, the e-cigarette’s manufacturer, for information that might indicate why its product is so appealing to young people. And Gottlieb said that while much of the focus is on that e-cig, other brands have similar characteristics.

The announcement about the crackdown came a week after lawmakers urged the FDA to be more aggressive in discouraging e-cigarette use among minors.