Childhood obesity rates in the United States have increased over the past 14 years, according to a recent study Reuters reported.

Researchers found that all classes of obesity have increased since 1999. They also saw an upward trend in the more severe forms of obesity -- those in which children have body mass index 120 to 140 percent higher than their peers.

"An increase in more severe forms of obesity in children is particularly troubling," Asheley Cockrell Skinner, lead author of the study and assistant professor of pediatrics in the University of North Carolina's 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."

For the study, Cockrell and colleagues analyzed data collected from more than 26,600 children between the ages of 2 and 19 from 1999 to 2012.

The findings are in contrast with a recent analysis by the U.S. Centers for Disease Control and Prevention that found a sharp drop in preschool obesity rates over the past decade, Reuters report. Both studies used data from the National Health and Nutrition Examination survey.

"The earlier study examined only the last decade, while we make use of all available years -- from 1999 to 2012. In 2003, there was an unusual uptick in obesity among young children, which led to the appearance of a significant decline," Skinner said in a statement. "However, when we look at the bigger picture, that change is not there."

The main message of the recent study is that childhood obesity rates have not improved.

"I don't want a study like the previous one to change the national discourse," Skinner told Reuters.

"One of the most important messages is whether we have an environment that allows for activity and encourages a healthy diet for all children, regardless of their weight," Skinner added.

The findings were recently published in JAMA Pediatrics.