The nation's lead public health agency last week focused its attack on electronic cigarettes on the issue of advertising, saying too many youths see the ads.

There are bans on TV commercials and some other types of marketing for regular cigarettes, but there are no restrictions on advertisements for e-cigarettes.

About 7 out of 10 youths said they'd seen the ads, mostly in stores, according to survey results released Tuesday by the Centers for Disease Control and Prevention.

About 22,000 high school and middle school students were asked in 2014 whether they'd seen e-cigarette ads - in stores, online, or in magazines, movies, and television.

The report doesn't prove advertising is causing more youths to pick up e-cigarettes, and a trade group said the survey was flawed. But CDC officials worry e-cigarettes may hook a new generation on nicotine and lead some to take up cigarettes, possibly reversing a long and gradual decline in smoking rates.

"Unfettered marketing of e-cigarettes has the potential to compromise decades of progress," said Brian King, a CDC expert on smoking issues.

Youth e-cigarette smoking rates have risen at a similar trajectory as spending on advertising, CDC officials say. And researchers say advertising has been a big driver of youth smoking of traditional cigarettes. Cigarette sales to minors are banned nationwide; most states now ban the sale of e-cigarettes to those under 18.

The federal health agency has been taking an unusually hard stand against e-cigarettes at a time when scientists are still trying to determine whether they are harmful. E-cigarettes have been sold in the United States for only about the last nine years.

E-cigarettes heat liquid nicotine into an inhalable vapor. Users get nicotine but not the thousands of chemicals, tar, or odor of regular cigarettes.

Scientists say nicotine is highly addictive and can be harmful for the developing brain. CDC director Tom Frieden said e-cigarettes could be a benefit if they help adult smokers quit but children should not be using them.

"They are not harmless," Frieden said Tuesday in a call with reporters.

The Smoke-Free Alternatives Trade Association - an e-cigarette industry trade group - criticized the survey and the CDC's stance.

"The CDC continues to mislead the public about the benefits of vapor products as far less harmful alternatives to smoking," the group's executive director, Cynthia Cabrera, said in a statement. "The CDC also fails to mention that teens are exposed to many other adult issues on the Internet, TV, and movies, such as violence, sex, and alcohol."

Cabrera said her group supported age restrictions on e-cigarette sales.

Though smoking of traditional cigarettes has plummeted to about 9 percent among high school students, e-cigarette use has become more common. Last year, about 13 percent said they'd used e-cigarettes in the previous month.

The U.S. Food and Drug Administration has been working on new rules for e-cigarettes and other, newer tobacco products. The regulations - first proposed in 2014 and currently under White House review - are not expected to contain advertising restrictions.

Advocates want limits. "It's time for the White House to issue a strong final rule and end this unregulated experiment that threatens our kids," Matthew L. Myers, president of the Campaign for Tobacco-Free Kids, said in a statement.