The US Food and Drug Administration has issued warning letters to tobacco companies that sell illegal flavored cigarettes via their websites to customers in the United States, it announced Friday.

The letters warn the companies to stop marketing and selling cigarettes enhanced with candy or fruit flavors and that are thought to encourage young people to take up smoking.

Teenagers are three times more likely to smoke flavored cigarettes than adults over age 25, studies have shown. Banning flavored tobacco could help keep an average of 3,600 teenagers per day from having their first cigarette, according to the FDA.

In their warning letters, the FDA says it will take action to bring the products into compliance with the law and, if necessary, impose additional regulatory actions, even though it can't impose an outright ban on cigarettes.

"FDA takes the enforcement of this flavored cigarette ban seriously," said Lawrence R. Deyton, director of FDA's Center for Tobacco Products in a statement. "These actions should send a clear message to those who continue to break the law that FDA will take necessary actions to protect our children from initiating tobacco use."

Flavored cigarettes were targeted under the Tobacco Control Act in June 2009 when the FDA was given authority to reign in the tobacco industry by banning some types of cigarettes, including grape- and clove-flavored cigarettes. The ban did not include menthol-flavored cigarettes.

Earlier this month, Canada banned the manufacture, importation and selling of most flavored cigarettes, but the ban also does not include menthol flavored cigarettes.