The U.S. Food and Drug Administration issued a warning letter Monday to the popular e-cigarette maker Juul for illegally advertising its product as a safer alternative to cigarettes and threatened to fine the company or seize its products if it does not correct its marketing.

The California-based company promoted Juul pods as a “modified risk tobacco product” — a legal designation — without the appropriate FDA order in effect, the letter stated.

The agency said that the law clearly states that before a company can market a product as reduced risk, it must first provide scientific evidence that the product is less harmful.

"Juul has ignored the law, and very concerningly, has made some of these statements in school to our nation’s youth,” acting FDA Commissioner Ned Sharpless said in a statement.

The warning letter included several statements that were presented during a July congressional hearing on Juul. According to testimony, a Juul representative told students in a school presentation that its e-cigarette “was much safer than cigarettes,” that “the FDA would approve it any day,” and that a student "... should mention Juul to his [nicotine-addicted] friend … because that’s a safer alternative than smoking cigarettes, and it would be better for the kid to use.”

The company has 15 days to provide a written response outlining the steps it took to correct its actions including discontinuing any misleading promotions, labeling, sale and distribution of the tobacco products.

The agency also sent a second letter asking for more information about issues raised during the congressional hearing about Juul’s outreach and marketing practices aimed at students, Native American tribes, health insurers, and employers.

The FDA has requested that Juul explain why it uses nicotine salts to mask the harshness of the chemical and why it uses a nicotine concentration of 5% in its products, which the agency is concerned will increase their addictive nature. The e-cigarette company has 30 days to comply.

The warning comes as health officials have recommended people avoid using e-cigarettes altogether while they investigate the cause of hundreds of vaping-related lung illnesses and at least five deaths that have been linked to vaping.

In August, a Delaware County teen who had been vaping the equivalent of a half-pack of cigarettes a day for over a year, including using an e-cigarette with cannabis, was hospitalized in critical but stable condition on a breathing machine after contracting a serious vaping-related lung illness.