GENEVA (Reuters) -- Twenty to 30 percent more young children may be overweight than previously thought according to new growth standards, the World Health Organization (WHO) said on Thursday. The UN agency said it hoped parents and pediatricians would begin applying its new guidelines -- which for the first time lay down optimal body mass index charts -- to halt "the increasing epidemic of childhood obesity."

The standards also aim to help identify malnourished youngsters, which it said was an underestimated problem.

The WHO -- which has declared war on poor diets blamed for rising obesity -- has estimated that at least 20 million children under five years and one billion adults worldwide are overweight.

Another 170 million children are underweight, three million of whom die each year as a result of malnutrition.

"We now have a much more precise tool to monitor child growth and identify both under-nutrition and over nutrition," Denise Costa Coitinho, director of WHO's department of nutrition for health and development, told a news briefing.

"This has life-long consequences because chronic diseases in later life like diabetes and cardiovascular diseases can be largely prevented with a good start in life," she added.

The WHO's new standards resulted from a nearly 10-year study which found that a child's growth is influenced more by environmental factors than genetics up to the age of five.

Mercedes de Onis, who coordinated the study, said that the new standards were "substantially different" from the 1977 charts established by the U.S. National Health Center Statistics, used by more than 100 countries.

She said that the WHO study showed there was a "range of 20 to 30 percent higher prevalence of overweightness" when compared with that U.S. standard.

This varied according to the country, and child's age group and sex.

To compile the global study, researchers tracked more than 8,000 children in Brazil, Ghana, India, Norway, Oman, and the United States from birth to age five, measuring them 21 times.

The new WHO growth charts include universal guidance for parents and health workers on healthy ranges of weight-for-age, height-for-age, and weight-for-height.