All classes of obesity in U.S. children have increased since 1999, according to a new study released Monday by the U.S. journal JAMA Pediatrics that contradicts a recent report that showed a decline in obesity among two to five years old in the last decade.

The new study, led by University of North Carolina (UNC) researchers, said what's "perhaps most troubling" is the upward trend in the more severe forms of obesity in which children have a body mass index (BMI) that is 120 to 140 percent higher than their peers.

"An increase in more severe forms of obesity in children is particularly troubling," lead author Asheley Cockrell Skinner, assistant professor of pediatrics in the UNC School of Medicine, said in a statement.

"Extreme obesity is more clearly associated with heart disease and diabetes risk in children and adolescents, and is more difficult to treat."

These findings are based on a new analysis of data collected from 26,690 children aged 2 to 19 from 1999 to 2012 as part of the National Health and Nutrition Examination Survey.

For the purposes of the study, "overweight" was defined as BMI greater than or equal to the 85th percentile for age and sex, and "obesity" was BMI greater than or equal to the 95th percentile.

The more severe forms of obesity, Class 2 and Class 3 obesity, were defined as a BMI greater than 120 percent of the 95th percentile for Class 2 and greater than 140 percent of the 95th percentile for Class 3.

Using these definitions, the study found that 17.3 percent of U. S. children aged 2 to 19 were obese in 2011-2012. At the same time, 5.9 percent met criteria for Class 2 obesity while 2.1 percent met that for Class 3 obesity.