E-cigarettes, widely promoted as an alternative to smoking, are actually attracting young people who might not otherwise have smoked tobacco products, a new U.S. study suggested Monday.

E-cigarettes are thought by some to be responsible for a decline in American youth cigarette smoking, but researchers from the University of California, San Francisco (UCSF) indicated the reality is the opposite.

The USCF researchers concluded that many kids who went on to smoke regular cigarettes may not have used nicotine at all if e-cigarettes did not exist.

"We didn't find any evidence that e-cigarettes are causing youth smoking to decline," said lead author Lauren Dutra of the UCSF.

"While some of the kids using e-cigarettes were also smoking cigarettes, we found that kids who were at low risk of starting nicotine with cigarettes were using e-cigarettes," Dutra said. "Recent declines in youth smoking are likely due to tobacco control efforts, not to e-cigarettes."

The findings, published in U.S. journal Pediatrics, built on a growing body of evidence that adolescents who start with e-cigarettes are more likely to subsequently smoke traditional cigarettes.

For the study, researchers examined data from more than 140,000 middle and high school students who completed a U.S. government tobacco survey between 2004 and 2014.

They found that cigarette smoking among U.S. adolescents declined during that decade, but did not decline faster after the advent of e-cigarettes in the U.S. between 2007 and 2009.

In fact, combined e-cigarette and cigarette use among adolescents in 2014 was higher than total cigarette use in 2009, according to the study.

"E-cigarettes are encouraging -- not discouraging -- youth to smoke and to consume nicotine, and are expanding the tobacco market," said senior author Stanton Glantz, UCSF professor of medicine and director of the UCSF Center for Tobacco Control Research and Education.

E-cigarettes are battery-powered devices that look like cigarettes and deliver an aerosol of nicotine and other chemicals.

In August 2016, the U.S. Food and Drug Administration restricted e-cigarette purchases to adults ages 18 and older.

The FDA will also require a warning label on e-cigarettes, starting August 2018, regarding the addictive nature of nicotine.

However, the FDA's ruling does not regulate advertising or flavors, and e-cigarettes continue to be sold in flavors that appeal to youth, the UCSF researchers said.