Chicago, Nov 16: Just days after the US city of Chicago took eight online retailers to court for illegally selling e-cigarettes to underage residents, federal agencies confirmed astonishing increases in nationwide youth use of e-cigarettes.According to the 2018 National Youth Tobacco Survey, released on Thursday by US Food and Drug Administration (FDA) and the Centres for Disease Control and Prevention (CDC), about 3.6 million of US middle and high school students are currently using e-cigarettes, about 1.5 million more than a year before. From 2017 to 2018, there was a 78 per cent increase in current e-cigarette use among high school students and a 48 percent increase among middle school students.In an open letter issued by FDA Commissioner Scott Gottlieb, he described youth use of e-cigarettes as an "epidemic." "These increases must stop. And the bottom line is this: I will not allow a generation of children to become addicted to nicotine through e-cigarettes," he wrote.The FDA chief vowed to take all necessary actions to stop these trends from continuing.US federal agencies now plan to prohibit the sale of sweet-flavoured electronic cigarette liquid at convenience stores and gas stations. Vaping products, including e-liquids, may now be sold only in stores that can verify the age of customers. They may be sold online only when there is more stringent age verification.E-cigarettes are devices that heat a liquid into an aerosol the user inhales. The liquid usually has nicotine and flavouring in it, and other additives.Earlier this week, Chicago filed lawsuits against eight online retailers, mostly based in Florida and California, for selling e-cigarette products directly to Chicago residents under the age of 21.