It says the number of children under five who are underweight has remained virtually unchanged since 1990, despite a target to reduce the number affected.

Half of all the under-nourished children in the world live in South Asia, Unicef reported.

And it said poor nutrition contributes to about 5.6 million child deaths per year, more than half the total.

One of the UN's Millennium Development Goals is to eradicate extreme poverty and hunger by 2015, which would mean halving the proportion of children who are underweight for their age.

But Unicef warned that the world was not on track to meet that goal.

In total, 27% of children under five in developing countries do not have enough to eat - around 146 million.

In 1990, the figure stood at 32%.

Food fortification

In South Asia, Bangladesh, India and Pakistan account for half of all the world's underweight children. About 47% of under-fives in India are underweight.

Eastern and Southern Africa, where famine regularly occurs, have made little progress - with 29% of children undernourished.

In East Asia, China has reduced the number of underweight children by an average of 6.7% per year since 1990, but other countries in the region are lagging behind.

Only two areas in the world - Latin America and the Caribbean where 7% of children are underweight, and the East Asia and Pacific region where 15% are affected - are on target to meet the MDG.

Central and Eastern Europe and the Commonwealth of Independent States have the world's lowest childhood underweight figures, at just 5%.

As in industrialised countries, there are disparities, with low birth weight is more common among the poorest and among ethnic minorities.

'Escape'

Vitamins and minerals are key to children's development, and deficiencies can increase the risk of common diseases.

For example, a lack of iodine in household diets leaves 37 million newborns vulnerable to learning disabilities every year.

Unicef is calling for range of measures including things as simple as providing vitamin A capsules, and fortifying foods with iron and iodine.

It also wants child nutrition to be a central component of national policies and budgets, providing better information on nutrition and better resources for families.

There should also be a special focus on the child's first two years of life and better promotion of breastfeeding.

Ann Veneman, executive director of Unicef, said: "The lack of progress to combat malnutrition is damaging children and nations.

"Few things have more impact than nutrition on a child's ability to survive, learn effectively and escape a life of poverty."

She warned that the figures did not tell the full story.

"For every visibly undernourished child, there are several more battling a hidden nutritional crisis."