The report, released Thursday, said current use of flavored e-cigarettes - defined as use in the past 30 days-had increased among high school students since 2014 and among middle school students since 2015.

Public health officials have called youth vaping an epidemic. The rates of eighth, 10th and 12th graders who use e-cigarettes continued to rise in 2019 and doubled from 2017 to 2019, according to research released last month. The United States is also facing an outbreak of lung injuries linked to vaping, many of them among young people.

"The high prevalence of flavored tobacco product use among middle and high school students is a concern because flavors can increase the appeal of tobacco products to youths, promote youth initiation of tobacco products, and result in lifelong tobacco product use," the report said.

Since 2009, federal regulations have prohibited flavored cigarettes other than tobacco or menthol, but no such federal prohibition applied to other tobacco products. In the last few weeks, several states and cities around the United States have restricted sales of e-cigarette flavors, which can include fruit, chocolate and candy flavors. The FDA is also moving to clear the market of e-cigarette flavors.

he new report said 3.15 million middle and high school student tobacco users -- nearly two-thirds of them -- said they used any type of flavored tobacco products in 2018. E-cigarettes were the most commonly used type of flavored tobacco.

The analysis, based on data from the National Youth Tobacco Survey, found that use of flavored hookah pipe tobacco among middle and high school students had declined from 2014 to 2018. The use of flavored smokeless tobacco, cigars, pipe tobacco and menthol cigarettes decreased among high school students during that time, the report said.

The report said the increase in flavored e-cigarette use might be because of the recent popularity and higher market share of e-cigarettes shaped like a USB flash drive, such as Juul.

"These products can be used discreetly, have a higher nicotine content than earlier generation e-cigarettes, and are available in flavors that appeal to youths, the report said.

"These attributes might play a role in sustained use; research shows the majority of youths and young adults who reported ever using Juul also reported being current Juul users," it added.

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.

In 2018, Juul announced it would stop selling most flavors in retail stores, including convenience stores and vape shops, as part of a plan to restrict access among minors.