The last few decades have seen a drop in cancer death rates driven largely by a reduction in smoking rates. However, cancer is still a major killer and the leading cause of death in 21 states, a new report says.

In those states, cancer has overtaken heart disease as the number one cause of death, reports American Cancer Society.

Heart disease deaths, although dropping at a steeper rate than cancer deaths in the last few decades, remains the number one killer nationwide over cancer, but only by a slim margin, the society reports.

Cancer has overtaken cardiovascular disease in certain segments of the population, including Asians, Hispanics and the 40-to 79-age group, it says.

"We're gratified to see cancer death rates continuing to drop," says Otis W. Brawley, M.D., chief medical officer of the American Cancer Society. . "But the fact that cancer is nonetheless becoming the top cause of death in many populations is a strong reminder that the fight is not over."

Along with early detections and treatment, a reduction in smoking is cited as the leading driver in the reduced cancer death rate in the U.S.; since 2009, the percentage of Americans who indulged in smoking dropped from 21 percent to 16.8 percent.

However, lung cancer results in one of four deaths from cancer, making the leading cancer killer.

Still, cancer deaths are dropping overall, the society's report noted.

"Over the past decade, cancer mortality dropped by 1.8 percent per year in men and 1.4 percent per year in women, driven by continued decreases in death rates for the four major cancer sites: lung, breast, prostate, and colon/rectum," the cancer society said in a statement.

In another statement, the American Society for Clinical Oncology, representing cancer specialists, cited progress in cancer prevention, surgery, chemotherapy, radiations and other treatments made possible by a national investment in research on cancer.

"Every cancer survivor is living proof of its progress," the group said.

However, despite advances in research and treatment, the American Cancer Society predicts almost 1.7 new cases of cancer will be diagnosed this year, and around 600,000 deaths will result.

"Cancer is in fact a group of more than 100 diseases, some amendable to treatment; some stubbornly resistant," Brawley says.