U.S. regulators on Tuesday issued warnings to 13 companies selling e-cigarette liquids for using child-friendly images in their packaging, in the latest crackdown aimed at preventing tobacco sales to minors.

The packaging resembles that of juice boxes, candy or cookies, the U.S. Food and Drug Administration and the Federal Trade Commission said, noting a recent increase in the number of reports to poison control centers.

"No tobacco products should be marketed in a way that endangers kids â€" especially by using imagery that misleads them into thinking the products are things they'd eat or drink," FDA Commissioner Scott Gottlieb said in a statement.

The FDA has made several sweeping moves in the past few months, including setting a maximum nicotine level for tobacco products as the regulator attempts to combat tobacco and nicotine addiction.

E-cigarettes are handheld electronic devices that vaporize an "e-liquid" fluid typically including nicotine and a flavor component. They have been grabbing market share away from traditional tobacco companies, and are available in different flavors.

"It takes a very small amount of these e-liquids, in some cases less than half a teaspoon ... to [have] a fatal effect for a kid and even less than that to make them very, very sick," an agency executive said on a call with reporters.

The FDA cited examples including "One Mad Hit Juice Box," which resembles children's apple juice boxes and "Twirly Pop," which not only resembles a Unicorn Pop lollipop, but comes with one.

Six of the letters issued were for dual violations where the products were illegally sold to minors online as well as packaged inappropriately.

"We don't have to wait until there's been an actual injury of a child we can take action if it's likely to cause substantial injury," Acting Federal Trade Commission Chairman Maureen Ohlhausen said.

The latest warnings come a week after the FDA sent 40 warning letters to companies on the sale of tobacco products to minors, particularly those made by Juul Labs Inc.

Twelve of the vendors issued warning letters on Tuesday did not immediately respond to Reuters' request for comment.

Nick Warrender of Lifted Liquids and E-Liquid Retail, which makes Vape Heads Sour Smurf Sauce, said the product had been pulled from the market and repackaged six months ago.

"We ... took a lot of money and steps to change the product to something that wouldn't be [as] child-appealing as the original packaging," Warrender said. "It seems to be a false narrative that they [regulators] are pushing."