A UN expert on food rights urged the international community on Thursday to act to prevent a looming crisis threatening several millions of people in west and central Africa.

"We must not wait until people are starving in order to act. The world must respond immediately to avert a full-scale food and nutrition crisis," Olivier De Schutter, the UN Special Rapporteur on the Right to Food, told reporters.

The crisis would be triggered by drought, poor harvests and rising food prices, he warned during a visit to Ghana.

Countries affected so far by drought are Chad, Mali, Mauritania and Niger while there are concerns over Burkina Faso, Senegal and northern parts of Nigeria and Cameroon, he said.

Six million people in Niger, 2.9 million in Mali and 700,000 in Mauritania are affected, he said.

"In Chad and Mauritania there is a grain deficit of more than 50 percent compared to last year," he said.

"Due to their greater nutritional needs, children face the highest risk of mortality linked to malnutrition, followed by pregnant and lactating women and adolescent girls."

De Schutter said the two regions were currently in the immediate post-harvest period when food prices should drop significantly but in some cases they continue to rise.

In Niger, millet prices were 37 percent higher in November 2011 than a year earlier, and other key cereals are up to 40 per cent higher than the regional five-year average.

"The (drought) season will come earlier and last longer than usual. This will leave the Sahel hugely reliant on food imports, which will have to be acquired at sky-high prices on the international market.

"This could spell disaster for millions of people whose food needs will rise as their purchasing power plummets," he said.

During his visit to Ghana, De Schutter will meet farmers and some organisations fighting against hunger and promoting food production.