Electronic cigarettes have surpassed traditional smoking in popularity among teens, the government's annual drug use survey finds.

Even as tobacco smoking by teens dropped to new lows, use of e-cigarettes reached levels that surprised researchers. The findings marked the survey's first attempt to measure the use of e-cigarettes by people that young.

Nearly 9 percent of eighth-graders said they'd used an e-cigarette in the previous month, while just 4 percent reported smoking a traditional cigarette, said the report being released Tuesday by the National Institutes of Health.

Use increased with age: Some 16 percent of 10th-graders had tried an e-cigarette in the past month, and 17 percent of high school seniors. Regular smoking continued inching down, to 7 percent of 10th-graders and 14 percent of 12th-graders.

"I worry that the tremendous progress that we've made over the last almost two decades in smoking could be reversed on us by the introduction of e-cigarettes," said University of Michigan professor Lloyd Johnston, who leads the annual Monitoring the Future survey of more than 41,000 students.

E-cigarettes often are described as a less dangerous alternative for regular smokers who can't or don't want to kick the habit. The battery-powered devices produce vapor infused with potentially addictive nicotine but without the same chemicals and tar of tobacco cigarettes.

The survey didn't ask about repeat use, or whether teens were just experimenting with something new. But between 4 percent and 7 percent of students who tried e-cigarettes said they'd never smoked a tobacco cigarette, noted University of Michigan professor Richard Miech, a study senior investigator.

"They must think that e-cigarettes are fundamentally different," he said.

E-cigarettes began to appear in the U.S. in 2006 but this was the first year that the Monitoring the Future survey asked teens about them. The Centers for Disease Control and Prevention has estimated that during 2013, 4.5 percent of high school students had tried e-cigarettes during the prior month, a tripling since 2011.

The CDC reported last week that 10 states permit the sale of e-cigarettes to minors. The Food and Drug Administration has proposed regulating e-cigarettes, including banning sales to minors; there is no timetable for final rules.

Other findings from the survey, funded by NIH's National Institute on Drug Abuse:

— Marijuana use appeared to level off after recent increases, with 6.5 percent of eighth-graders reporting past-month use, 17 percent of 10th-graders and 21 percent of 12th-graders. Nearly 6 percent of 12th-graders reported daily use.

— Fewer teens are trying synthetic marijuana, highly dangerous drugs known by such names as K2 and Spice. About 6 percent of seniors said they had used fake pot this year, down from 8 percent last year and 11 percent in 2012.

— Abuse of prescription painkillers is dropping. Six percent of high school seniors reported using the narcotics without medical supervision in the past year, down from 9.5 percent in 2004.

— Nearly 1 in 5 12th-graders reported binge drinking, defined as five or more drinks in a row in the previous two weeks. That's down from 1 in 4 high school seniors in 2009.