LOS ANGELES — Although teen smoking has declined, more than 3 million high school students and 600,000 middle school students still smoke cigarettes and are at risk of early lung and heart problems, according to a report issued Thursday by the U.S. surgeon general.

The smoking rates among teenage high school students have dropped from 27.5 percent in 1994 to 19.5 percent now, but the decline has slowed in recent years.

Nearly 90 percent of new smokers start before they turn 18 and three-quarters of high school smokers continue into adulthood, the report said. They are also more likely to get addicted because of their young age.

“The addictive power of nicotine makes tobacco use much more than a passing phase for most teens,” Surgeon General Dr. Regina Benjamin said in a statement. “We now know smoking causes immediate physical damage, some of which is permanent.”

Public health campaigns have raised awareness about the dangers of smoking and laws have made it more difficult for youths to buy cigarettes, but Benjamin said the U.S. needs to do more to keep young people from lighting up their first cigarette.

One of the challenges, according to the report, is the extensive marketing and advertising of cigarettes to young people. More than $1 million is spent every hour on marketing of tobacco products, the report said. The industry also targets teens through smokeless tobacco products.

The report, Preventing Tobacco Use Among Youth and Young Adults, is the first such publication since 1994 and can be read at Surgeongeneral.gov.