Around 1,200 warning letters have been sent by US health authorities to leading retailers for unlawfully selling cigarettes to children. Firms that have received these letters include CVS, Walgreen, Wal-Mart and Rite Aid.
The letters came after Food and Drug Administration inspectors carried out more than 27,500 compliance checks and began inspecting US tobacco product manufacturers in October -- the first time such facilities have ever been inspected by a federal public health agency.
White House spokesman Jay Carney said President Barack Obama, who has struggled to kick a long-standing smoking habit, was "committed to protecting children from cigarettes" and to stop youngsters from ever smoking.
The FDA letters followed "an aggressive inspection campaign and remind retailers of their legal responsibility to protect our children" under 18, he added.
"Most retail store owners follow the law and don't sell cigarettes to kids, but we're reminding those who don't that they have a responsibility to follow the law and that there are serious consequences if they fail to do so," Health Secretary Kathleen Sebelius said on the official White House blog.
She stressed that "keeping tobacco out of the hands of minors can have a huge impact on our nation's health now and in the future," noting that 20 percent of US high school students smoke.
"We know our campaign against tobacco is a 'winnable battle,'" she added.
Tobacco is the leading cause of preventable deaths in the United States, where about 3,450 youths aged 12-17 smoke their first cigarette each day, and an estimated 850 of them become daily smokers, according to the Centers for Disease Control and Prevention.
The news of the warning letters also came just days after a federal judge blocked the Obama administration's bid to place graphic warning labels on cigarette packs, ruling the move would violate the US Constitution's free speech protections.
Hinting at an appeal of the court decision, Carney said: "We are confident that Big Tobacco's attempts to stop these warnings from going forward will ultimately fail."