(CNN) The US Food and Drug Administration on Monday warned leading e-cigarette maker Juul Labs about illegally marketing its product as a safer alternative to cigarettes.

The FDA ordered Juul to respond within 15 working days with corrective actions and its plan to comply with federal law. The warning letter noted that failure to comply could result in fines, seizures or injunction.

"Regardless of where products like e-cigarettes fall on the continuum of tobacco product risk, the law is clear that, before marketing tobacco products for reduced risk, companies must demonstrate with scientific evidence that their specific product does in fact pose less risk or 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 Dr. Ned Sharpless said in a statement.

"We will continue to scrutinize tobacco product marketing and take action as appropriate to ensure that the public is not misled into believing a certain product has been proven less risky or less harmful," he said. "We've also put the industry on notice: If the disturbing rise in youth e-cigarette use continues, especially through the use of flavors that appeal to kids, we'll take even more aggressive action."

The FDA sent a separate letter to Juul "expressing concern, and requesting more information, about several issues raised in a recent Congressional hearing regarding JUUL's outreach and marking practices, including those targeted at students, tribes, health insurers and employers."

The agency requested documents related to marketing, including scientific evidence and data such as consumer perception studies "related to whether these statements and representations explicitly or implicitly convey that JUUL products pose less risk, are less harmful, present reduced exposure, are safer than other tobacco products or that the products are smoking cessation products."

In November, the FDA revealed that vaping had increased nearly 80% among high schoolers and 50% among middle schoolers since a year earlier. Public health experts have said that Juul has largely propelled the rise, commanding about 75% of the e-cigarette market in the United States.

"We believe you have a continuing responsibility to take action to address the epidemic of youth use of your products, some of which appears to have been a direct result of your product design and marketing campaigns, whether or not some of these practices have been discontinued," the FDA's separate letter to Juul said.

Juul has maintained that its products are intended to convert adult smokers to what it described in the past as a less-harmful alternative. In other communications, the company says it cannot make claims its products are safer, in line with FDA regulations.

"We are reviewing the letters and will fully cooperate," according to Ted Kwong, a Juul Labs spokesperson.

Calls for action against Juul

Last week, Rep. Raja Krishnamoorthi, chairman of the House Oversight Subcommittee on Economic and Consumer Policy, asked the FDA in a letter to take "appropriate enforcement action" against Juul. He said the agency should "protect the American public from the fraudulent and unapproved medical claims" made by the company.

Krishnamoorthi's letter followed a two-day hearing in July, after which the committee concluded that "JUUL appears to be violating FDA regulations against making unapproved express and implied claims that its product helps users stop smoking cigarettes and is safer than cigarettes."

At the subcommittee hearing in July, several people testified that the company was directly marketing to children in high school, to the Cheyenne River Sioux Tribe and to smoking cessation groups.

On Friday, the US Centers for Disease Control and Prevention announced that the agency is now aware of at least 450 possible cases of severe lung disease that could be caused by vaping across 33 states.

There have been at least five deaths across five states -- one each in Illinois, Oregon, Minnesota, Indiana and California -- linked to the illnesses.

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US Senate Democratic Whip Dick Durbin pointed out in a press conference on Monday how Sharpless' predecessor, Dr. Scott Gottlieb, called vaping among youth an "epidemic."

"That was his word, and it is an epidemic, and why the FDA refuses to act, I can not answer," Durbin said during the press conference.

After news of additional deaths spread on Friday, Durbin called on Sharpless to act on regulating e-cigarettes and flavors in the next 10 days. Durbin said he would call for Sharpless' resignation if he did not take action.

Then on Monday, Durbin said again that it's time Sharpless "either does something or resigns."

"This is a kids' toy, make no mistake about it, no matter what Juul tells you," he said. "We're naïve to believe that children aren't buying these and using these for purposes that are endangering their health. This ought to be closely regulated by the Food and Drug Administration. We are facing, in their words, an epidemic, taking the lives of children across the United States."

Durbin added during the press conference that public education around this issue is not enough and enforcement is needed.

"We have to send people undercover into these places that are selling these devices, and when we nail them selling them to people underage, they pay a heavy price for it. The word gets out quickly in the retail community," he said.

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In response to Durbin's criticism, FDA spokeswoman Stephanie Caccomo said in an emailed statement on Friday that the agency looks forward to "engaging with Senator Durbin, along with all members of Congress, on this ongoing and very serious situation."

"Getting to the bottom of this is a top priority for the agency and all of our federal and state partners. We are all working tirelessly to get as much information as possible about any products or substances used. We are leaving no stone unturned in following any potential leads and we're committed to taking appropriate actions as the facts emerge," the statement said in part. "The illnesses under investigation involve the broader use of vaping products — including those being used with substances like THC; e-cigarettes are considered one type of vaping product. With respect to our work to tackle the youth e-cigarette epidemic, we remain committed to our oversight of e-cigarettes and to keeping them out of the hands of youth."