(Reuters) - The U.S. Food and Drug Administration and the Federal Trade Commission on Friday issued warning letters to four flavored e-liquid product makers, stepping up pressure on firms using social media influencers to promote their products.

Social media marketing has come under increased scrutiny as part of attempts by health regulators to control the marketing of tobacco products, particularly the newest generation of products including the wildly popular Juul e-cigarettes.

Posts on social media on behalf of Solace Technologies LLC, Hype City Vapors LLC, Humble Juice Co LLC and Artist Liquids Laboratories LLC had content touting the flavored e-liquid products by not including the required nicotine warning statement, the agency said.

“These letters are a reminder that companies who use social media influencers to promote their products must comply with all applicable advertising requirements,” Andrew Smith, director of the FTC’s Bureau of Consumer Protection, said.

More than 100 public health and anti-tobacco organizations have called for swift action from social media platforms, including Facebook Inc, Instagram, Twitter and Snap Inc, after Reuters recently reported here how cigarette maker Philip Morris International Inc has used young personalities on Instagram to sell a new "heated tobacco" product called IQOS.

More than 3.6 million middle and high school students across the United States were e-cigarette users in 2018, according to the National Youth Tobacco Survey jointly conducted by the FDA and the U.S. Centers for Disease Control and Prevention.

The data also showed that youth who used e-cigarettes were also using them more frequently, and they were using flavored e-cigarette products more often in 2018, than in 2017, the agency said.