The number of American teens and preteens smoking electronic cigarettes doubled in 2012 over the year before -- a trend US health authorities called "deeply troubling" on Friday.

Some 1.78 million US middle and high school students -- around 11 to 18 years old -- smoked so-called e-cigarettes in 2012, the Centers for Disease Control and Prevention (CDC) said in a new study.

An estimated 160,000 of them had never even smoked a conventional cigarette, a fact it found worrying, because "the overall impact of e-cigarette use on public health remains uncertain," the CDC said.

"Nicotine is a highly addictive drug. Many teens who start with e-cigarettes may be condemned to struggling with a lifelong addiction to nicotine and conventional cigarettes," said CDC Director Tom Frieden.

E-cigarettes are battery-powered and deliver doses of nicotine and other additives via aerosol.

Around 10 percent of high school students reported trying an "e-cigarette" at least once in 2012, double the figure from the year previous. And just under three percent reported smoking e-cigarettes regularly in 2012, up from just 1.5 percent in 2011.

Of those who tried or used e-cigarettes, just over seven percent had never smoked a conventional cigarette.

Among middle schoolers, experimentation with the product rose from 1.4 percent to 2.7 percent over the same period.

"The increased use of e-cigarettes by teens is deeply troubling," Frieden said.

CDC smoking and health office director Tim McAfee noted that "about 90 percent of all smokers begin smoking as teenagers."

"We must keep our youth from experimenting or using any tobacco product. These dramatic increases suggest that developing strategies to prevent marketing, sales, and use of e-cigarettes among youth is critical."

The US government is due to announce in October its plans for regulating electronic cigarettes. For the moment, the laws are different according to each state, though a number of them have already forbidden their sale to minors.

You must be at least 18 to purchase conventional cigarettes in the US.

The CDC study was carried out as a questionnaire sent to 20,000 students, part of an annual report on tobacco use among young people.

[Image via Agence France-Presse]