MONDAY, Sept. 9 (HealthDay News) -- While the rate of obesity among U.S. children seems to have leveled off, the number who are "severely obese" continues to rise, according to the American Heart Association.

Some 5 percent of children and teens now fall into this category, putting them at high risk for premature heart disease and type 2 diabetes, the researchers report in an American Heart Association scientific statement published in the Sept. 9 online edition of Circulation.

"We are defining a new class of pediatric obesity called 'severe' obesity," said lead researcher Aaron Kelly, an associate professor of pediatrics at the University of Minnesota Medical School in Minneapolis.

"This type of obesity is different than traditional obesity and overweight in children," he said. "It is an extreme form of obesity."

Children are considered severely obese if they have a body mass index (BMI) above the 95th percentile for their sex and age, or a BMI of 35 or higher. BMI is a measure of weight in relation to height, and a child in the 95th percentile weighs more than 95 percent of other children the same sex and age.