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 and stop 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, including myblu and KandyPens, have similar characteristics.

The announcement about the crackdown came a week after health organizations and lawmakers urged the FDA to be more aggressive in discouraging e-cigarette use among minors. The Campaign for Tobacco-Free Kids, Truth Initiative, American Academy of Pediatrics, American Cancer Society Cancer Action Network, American Heart Association and American Lung Association, sent a letter to Gottlieb warning that progress against smoking is “at serious risk of being reversed” because of the agency’s failure to take action against products that appeal to youth.

Separately, a group of 11 Democratic senators, led by Richard Durbin of Illinois, complained to Gottlieb and Juul Labs Chief Executive Kevin Burns about youth use of the product.

On Tuesday, the health and anti-tobacco groups praised the FDA’s new steps but said they didn’t go far enough.

“The FDA needs to accelerate its actions when it comes to regulating e-cigarettes like Juul and remove flavors known to entice youth,” said Dave Dobbins, chief operating officer of Truth Initiative, a tobacco-control group. “Keeping e-cigarettes on the market without first evaluating them is putting an entire generation of young people at risk of addiction.”

Juul Labs agreed in a statement that sales to minors were “unacceptable” and noted that it already has “programs to prevent and, if necessary, identify and act upon these violations at retail and online marketplaces.” It said it will announce “additional measures in the coming days” as it works with the FDA and lawmakers to keep its e-cigarettes away from young people.

As part of a comprehensive tobacco policy announced last summer, Gottlieb vowed to reduce the nicotine in conventional cigarettes to nonaddictive levels and said he believes e-cigarettes could be an important tool in helping adult smokers switch to less harmful nicotine-delivery products. But that policy delayed for several years a requirement that e-cigarette makers get agency approval for their products. Health groups have sued the agency over the delay.

Gottlieb reiterated Tuesday that e-cigarettes could be helpful for addicted adult smokers. However, he said their viability “is severely undermined if those products entice youth to start using tobacco and nicotine.”