The marketing of unhealthy foods to children has proven "disastrously effective", driving obesity by using cheap social media channels to promote fat-, salt- and sugar-laden foods, the World Health Organisation's Europe office said.
The United Nations health agency called for tighter controls on such marketing, saying tougher regulations were crucial to winning the fight against childhood obesity.
"Children are surrounded by adverts urging them to consume high-fat, high-sugar, high-salt foods, even when they are in places where they should be protected, such as schools and sports facilities," said Zsuzsanna Jakab, director of the WHO's regional unit for Europe.
The promotion of foods high in saturated and trans-fats, sugars and salt has for years been recognised as a significant risk factor for obesity in children and for diet-related chronic diseases such as heart disease and some cancers later in life.
In a report on food marketing, WHO Europe said the food industry increasingly uses cheap new marketing channels such as social media and smart phone apps to target children.
Television remains the dominant form of advertising and a large majority of children and adolescents watch TV on average for more than two hours a day, it said.
"Overweight is one of the biggest public health challenges of the 21st century: all countries are affected to varying extents, particularly in the lower socioeconomic groups," Jakab said in a foreword to the report.
And the picture is not improving, she added. Data from the WHO's Childhood Obesity Surveillance Initiative show that, on average, one child in every three aged 6 to 9 years is overweight or obese.
Jakab also said recent data suggest children become obese not just because they watch TV instead of being active but also because of exposure to advertising and other marketing tactics.
Leading categories of advertised foods are soft drinks, sweetened breakfast cereals, biscuits, sweets, snacks, ready meals and fast food outlets, the WHO report said.
"Unfortunately, marketing unhealthy food to children has been proven to be disastrously effective," the report said. "Whereas adults are aware when they are being targeted, children are unable to distinguish, for example, between adverts and cartoons. This makes them particularly receptive and vulnerable to messages that lead to unhealthy choices."
WHO Europe said that, while all 53 member states of its European region have signed up to restrictions on the marketing of unhealthy foods to children, most rely on general advertising regulations that do not specifically address the promotion of high-fat, -salt or -sugar products.
More comprehensive approaches -- via either legislation, self-regulation or co-regulation -- have only been adopted in Denmark, France, Norway, Slovenia, Spain and Sweden, it said.