The United Nations agency, issuing its first such guidelines, said under-fives should also be physically active and get adequate sleep to help develop good lifelong habits and prevent obesity and other diseases in later life.

Sedentary screen time would include watching television or videos and playing computer games.

"Healthy physical activity, sedentary behaviour and sleep habits are established early in life, providing an opportunity to shape habits through childhood, adolescence and into adulthood," the WHO said in the guidelines to member states.

Children between one and four years old should spend at least three hours in a variety of physical activities spread throughout the day, it said. Infants under one should interact in floor-based play and avoid all screens, it said.

Being inactive is a "leading risk factor" for mortality and fuels the global rise in overweight and obesity, the WHO said.

In a report two years ago, the WHO said the number of obese children and adolescents worldwide had jumped tenfold to 120 million in the past 40 years and that the rise was accelerating in low- and middle-income countries, especially in Asia.

Excessive weight can lead to diseases including diabetes, hypertension and some forms of cancer, it said.

KEEP ACTIVE

Early childhood is a period of rapid physical and cognitive development during which habits are formed and family lifestyle routines are adaptable, it said in the guidelines, drawn from evidence in hundreds of studies, many from Australia, Canada, South Africa and the United States.

"Sedentary behaviours, whether riding motorised transport rather than walking or cycling, sitting at a desk in school, watching TV or playing inactive screen-based games are increasingly prevalent and associated with poor health outcomes," the WHO said.

Chronic insufficient sleep in children has been associated with increased excessive fat accumulation as measured by body mass index (BMI), it said.

Shorter sleep duration has been associated with more TV viewing and time spent playing computer games, it added.

"Improving the physical activity, sedentary and sleep time behaviours of young children will contribute to their physical health, reduce the risk of developing obesity in childhood and the associated non-communicable diseases in later life and improve mental health and well-being," the WHO said. (Reporting by Stephanie Nebehay Editing by Gareth Jones)