Its report warns parents are often unaware of the sheer volume of such adverts as they are precisely targeting children.
The WHO also criticised governments for failing to keep up with a revolution in the way people consume media.
Children's doctors said strict measures were needed to stop childhood obesity.
The report attacked the way some video bloggers - vloggers - get paid by junk food retailers to promote their food.
It quotes a US analysis that suggests vloggers are now more influential at promoting brands than film or TV because of perceived authenticity.
It also raised concern about the way fast food chains encourage kids through their doors by making restaurants important locations in augmented reality games like Pokemon Go.
And said data on children - their age, location, likes and preferences - were being collected to target them with junk food adverts.
Dr Joao Breda, the WHO programme manager for nutrition, physical activity and obesity, told the BBC News website: "It is going digital very strongly and we know that existing models of regulation have holes and gaps that don't cover the needs of our children.
"We think it's huge, but parents don't know - sometimes they don't realise their children are being exposed.
"You could argue that is it more dangerous [than traditional media like TV]."
Some countries such as the UK have introduced rules to protect children from junk-food advertising such as bans during children's television, however, the report said regulation had "failed to keep up with the pace and scope of change in the media".
On Thursday, figures in England showed nearly one-in-five children were obese when they left primary school.
"With the obesogenic environment they are living in today, it really isn't a surprise," said Prof Russell Viner, from the UK's Royal College of Paediatrics and Child Health.
He added: "It is reprehensible that children are targeted online, through billboard advertising and on television as they watch their favourite shows."
Prof Viner said there needed to be "strict measures put in place to protect" children and that governments needed to act urgently to prevent "marketers cynically targeting children online, on our streets and on television".
Dr Alison Tedstone, chief nutritionist at Public Health England, said: "Our evidence review shows that all forms of advertising and marketing - including the use of characters, advergames and digital marketing - affect the balance of children's diets."
The organisation is currently reviewing what foods and drinks can be advertised to children.