Many Americans are failing to meet the minimum recommendations for exercise, although confusing guidelines are making it difficult to assess, researchers reported on Thursday.

Depending on which federal exercise recommendations are used, either about half or about two-thirds of Americans meet minimum goals, the team at the U.S. Centers for Disease Control and Prevention found.

"Additional efforts are needed to further increase physical activity," they concluded in the CDC's weekly report on death and disease.

"Approximately one third of U.S. adults did not report meeting minimum levels of aerobic physical activity as defined by the 2008 Guidelines."

Under guidelines released by the U.S. Health and Human Services Department in October, the minimum recommended aerobic physical activity is 150 minutes — two and a half hours — a week of moderate activity such as brisk walking, or 75 minutes a week of vigorous activity such as running.

Just under 65 percent of adults reached that goal, the CDC said.

The government's Healthy People 2010 objectives call for at least 30 minutes of moderate activity five days a week, or 20 minutes of vigorous activity three days per week. Only 49 percent of those surveyed met these goals.

For the study the CDC used a telephone survey of 399,000 U.S. adults.

The CDC said it is easier to meet overall goals of minutes of exercise per week.

"Existing scientific evidence cannot determine whether the health benefits of 30 minutes of activity, 5 days per week, are any different from the benefits of 50 minutes, 3 days per week," the report reads.

It may be less confusing to advise people to get exercise when they can, the CDC said.

The Institute of Medicine has stronger guidelines. A 2002 report from that group said people should exercise at least an hour a day to maintain health and weight.

About a third of Americans are obese and another one-third are overweight, the CDC says, and they are at higher than acceptable risk of heart disease, diabetes and cancer.