U.S. regulators slapped four e-cigarette companies with warning letters Friday for failing to follow federal advertising rules for tobacco on Instagram, Facebook and Twitter.

The Food and Drug Administration and Federal Trade Commission reprimanded Solace Technologies, Hype City Vapors, Humble Juice Co. and Artist Liquids Laboratories for failing to disclose the health and safety risks of vaping in postings by social media influencers who are paid to help market their products.

Regulators say the companies violated advertising rules and misbranded their products by failing to include the statement: "WARNING: This product contains nicotine. Nicotine is an addictive chemical," which the FDA has required since August.

"Given the significant risk of addiction, the failure to disclose the presence of and risks associated with nicotine raises concerns that the social media postings could be unfair or likely to mislead consumers," the agencies said in separate letters to all four companies. They also told the companies to review their marketing materials and postings by social media influencers to ensure they include the proper disclosures and warnings.

Regulators do not restrict companies from using this kind of marketing to promote e-cigarettes. However, the FDA requires them to include a warning statement saying the products contain nicotine and that nicotine is an addictive chemical.

"Years of progress to combat youth use of tobacco is now threatened by an epidemic of e-cigarette use by kids, and unfortunately research shows many youth are mistaken or unaware of the risks and the presence of nicotine in e-cigarettes," acting FDA Commissioner Ned Sharpless said in a statement. "That's why it's critical we ensure manufacturers, retailers and others are including the required health warning about nicotine's addictive properties on packages and advertisements — especially on social media platforms popular with kids."

Influencers are individuals who have a large number of followers and are often paid to help promote products on Twitter, Instagram and Facebook where it appears to be more authentic than an advertisement. Using social media is a popular form of e-cigarette advertising, though the practice is coming under scrutiny. Market leader Juul shuttered its accounts in the fall amid pressure from the FDA, which has an ongoing investigation into the company's marketing practices.

The FDA is trying to crack down on what it's calling a teen vaping "epidemic." High school seniors' use of e-cigarettes surged 78% last year, according to a federal survey.

Public health groups have called on regulators to restrict e-cigarette advertising. Philip Morris International's use of influencers to promote its new heated tobacco product iQOS came under fire after Reuters found one of the models was 21, violating the company's marketing code, and posted seductive photos in luxurious settings. The company in response suspended its social media campaign.

The four companies cited Friday have 15 working days to respond to concerns regulators raised and specify what actions they're taking to address them.

A Solace Vapor spokesman said all of its "internal packaging, marketing and nicotine warnings are compliant with FDA standards" and it will review and terminate influencers "who may not be compliant with our marketing practices." The other three companies did not immediately respond to CNBC's request for comment.