US consumer groups have again put the marketing of unhealthy foods to children in the spotlight and raised questions over the rigour of industry self-regulation.

A new study by Children Now has claimed to reveal the "ineffectiveness" of the food and drinks industry's voluntary "better for you" initiative to improve the nutritional quality of foods advertised to children.

The research found that, despite adhering to the "better for you" guidelines, 68.5% of all advertising to children from companies involved in the initiative is in the "lowest category" of nutritional quality - products which, according to the US Department of Health, should be consumed only on "special occasions, such as your birthday".

In contrast, the study found advertising of fruit and vegetables was "virtually non-existent".

"It is known that, to combat the childhood obesity epidemic, the balance of children's food marketing must shift away from unhealthy products," said Dr. Dale Kunkel of the University of Arizona, who conducted the study for Children Now.

"The industry has failed to do this on its own, which points to the need for government regulation."

The Grocery Manufacturers Association, the body that represents the US food industry, was unavailable for comment at time of press.