Four in five adolescents worldwide do not get enough physical activity, the World Health Organization said Friday, warning that girls especially need more exercise.

In its first ever report on global trends for adolescent physical activity, the UN health 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.

The report is based on data from surveys conducted between 2001 and 2016 of some 1.6 million students between the ages of 11 and 17 across 146 countries.

It found that 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 organised 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.

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

"We need to do more if we want to halt the rise in obesity in this age group and promote better levels of physical activity," Riley said.