Manipulative marketing to children by multinational fast food merchants must be banned to effectively fight childhood obesity, New Zealand researchers said Friday.

Lead researcher Dr Gabrielle Jenkin, of the University of Otago, said most children and parents would be familiar with offers of free toys at McDonalds, slogans such as "Open happiness" with Coke, and the use of licensed characters such as Spiderman or Spongebob Squarepants to promote junk food.

"Such marketing has been proven to increase children's requests for the advertised foods, their food preferences and ultimately their diets. For example, free toys, discounts and competitions promote brand loyalty and repeat purchases," she said in a statement.

The researchers called for an outright ban on junk food advertising to children under 16, as had been done in Norway.

In the absence of a ban, new rules would need to be added to the advertising codes around the use of persuasive techniques, as had been done in the United Kingdom, Australia and Ireland.

Jenkin said the ubiquitous marketing of energy-dense, nutrient- poor food and beverages had come under increasing public health scrutiny by international health organizations that had called on governments to monitor and address the problem.

In a study of persuasive marketing techniques on television, they found premium offers, the use of promotional characters, nutritional and health claims, and the themes of "taste" and "fun" were commonly used to promote unhealthy food to children.

"Addressing this issue would make a meaningful contribution to curbing the international obesity epidemic besieging children throughout the world," said Jenkin.

The World Health Organization had recommended the reduction of "both the exposure of children to, and the power of, marketing of foods," she said.