GENEVA - Four in five adolescents worldwide do not get enough physical activity, to the detriment of their health, the World Health Organization said on Friday, warning that girls especially need more exercise.

In its first report on global trends for adolescent physical activity, the UN agency stressed that urgent action was needed to get teens off their screens and moving more.

"We absolutely need to do more or we will be looking at a very bleak health picture for these adolescents," study co-author Leanne Riley told journalists ahead of the launch.

The report, which was published in the Lancet Child & Adolescent Health journal, is based on data from surveys conducted between 2001 and 2016 of nearly 1.6 million students between the ages of 11 and 17 across 146 countries.

It found 81 percent did not meet the WHO recommendation of at least an hour a day of physical activity such as walking, playing, riding a bike or taking part in organized sports.

This is worrying, since regular physical activity provides a host of health benefits, from improved heart and respiratory fitness to better cognitive function, making learning easier.

Exercise is also seen as an important tool in efforts to stem the global obesity epidemic.

'No improvements'

But despite ambitious global targets for increasing physical activity, the study found virtually no change over the 15-year-period it covered.

"We are not seeing any improvements," Riley said.

While the report does not specifically study the reasons for adolescent physical inactivity, she suggested that the "electronic revolution ... seems to have changed adolescents' movement patterns and encourages them to sit more, to be less active".

The report authors also pointed to poor infrastructure and insecurity making it difficult for adolescents to walk or cycle to school.

The study found that levels of physical inactivity among adolescents were persistently high across all regions and all countries, ranging from 66 percent in Bangladesh to 94 percent in South Korea.

"We find a high prevalence pretty much everywhere," lead author Regina Guthold said, noting that in "many, many countries, between 80 percent and 90 percent of adolescents (are) not meeting the recommendations for physical activity."

And the situation was particularly concerning for adolescent girls, with only 15 percent of them getting the prescribed amount of physical activity, compared with 22 percent for boys.

Agence France-Presse