The prevalence of smoking in the U.S. has fallen over the last five years, but not at a steady rate, according to a study by the Centers for Disease Control and Prevention.

The report finds that the percentage of adults who smoke fell from 20.9 percent in 2005 to 19.3 percent in 2010, a 1.6 percent drop that amounts to about 3 million fewer smokers.

The report comes as the Food and Drug Administration announced new regulations that will require tobacco companies to present additional warnings on the front of tobacco products. The law is calling for larger, graphic warning labels on cigarette packs starting September 2012.

While the results represent progress, more needs to be done in order to combat health risks posed by smoking, said CDC Director Dr. Tom Frieden.

The modest reduction seen between 2005 and 2010 is likely attributable to the proliferation of smoke-free environments, greater awareness of the dangers of smoking and increased cigarette prices.

But the rate of the decline between 2005 and 2010 is slower than in the previous five-year period, the CDC notes.

The report also showed while the percentage of adults who smoke 30 or more cigarettes per day is down by 4 percent, the percentage of those who smoke nine or fewer cigarettes daily increased from 16.4 percent in 2005 to 22 percent in 2010.

Fifty percent of adults who continue to smoke will die from smoking-related causes, according to the report.

Tobacco use remains the leading preventable cause of death and disease in the United States. Tobacco use and exposure to secondhand tobacco smoke kill an estimated 443,000 Americans each year.

Smoking costs the U.S. $193 billion per year in healthcare and lost productivity.

The tobacco industry spends almost $10 billion per year on advertising and promotion, and 72 percent of these dollars are spent on discounts to offset the costs of tobacco taxation and other policies.

The CDC said states need to step up their tobacco control policies.