FILE PHOTO: A sign advertising Juul brand vaping products is seen outside a shop in Manhattan in New York City, New York, U.S., February 6, 2019. REUTERS/Mike Segar

(Reuters) - The U.S. Food and Drug Administration warned Juul Labs Inc on Monday for marketing its e-cigarettes as safer than traditional cigarettes, the latest move by the agency to curb the use of vaping devices that have become extremely popular among teens.

Juul has already come under scrutiny for its marketing initiatives, including its use of social media influencers to promote its vaping devices, with the Federal Trade Commission launching an investigation last month.

"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," acting FDA Commissioner Ned Sharpless said in a statement. (bit.ly/2krQlGX)

The FDA’s warning letter also raises issues with certain statements made by those attending a July U.S. congressional hearing where a panel grilled Juul over a “holistic health education” camp it funded, as part of efforts to market its products directly to school-aged children.

According to a testimony at the hearing, a company representative speaking at a presentation in a school claimed that students “…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 uses a nicotine concentration of 5% in its products, which could potentially increase their addictiveness, and nicotine salts that are used to mask the harshness of nicotine, the agency said questioning their usage.

“We are reviewing the letters and will fully cooperate,” a Juul spokesman said.

The FDA has also asked Juul to provide a written response within 15 days outlining its plan to correct its violations and to provide requested documents and information within 30 days of the date of the letter.

Last year, the agency had requested documents from Juul to examine the high rates of youth use and had conducted a surprise inspection at Juul’s San Francisco headquarter and seized documents.