It said it would take several weeks for its scientists to make a final decision on whether the food was safe.

The move has fuelled a debate over the safety of genetically modified food in southern Africa, where 13 million people are facing the region's worst food shortage in a decade.

"In Zambia there are 2.4 million people at risk of starvation. For us to do the job that needs to be done we have to use bio-tech/GM food," James Morris, executive director of the World Food Programme, said in an interview.

The WFP has halted distribution of 15,000 tonnes of maize suspected to have GM content, and has about 7,000 tonnes of non-GM food left in the country to hand out.

"We will run out shortly, although we have a huge amount of food throughout the country to use," Morris said.

"We've essentially never had anyone turn us down over the 40 year history of WFP. But a country has the right to say 'No, we don't want that'," he added.

He said he would discuss the GM ban with Zambian President Levy Mwanawasa during a tour of southern Africa next week.

The United States is the largest contributor of food aid to the region, and is the world's biggest producer of crops that are genetically modified to make them resistant to pests or to withstand herbicides that kill nearby weeds.

Morris said the agency sources food aid from many countries - including the United States, Japan, Canada, Brazil and South Africa - that use agricultural biotechnology.

"I would say that 75 percent of the food we have to work with right now comes from sources that produce GM/biotech commodities," Morris said.

THIS STUFF IS SAFE

"There is no evidence that it is the least bit harmful to anyone's health. It meets all the standards we have...This stuff is safe," he added.

Washington criticised African leaders and environmental groups on Friday for shunning GM food aid, saying their actions endangered millions of starving people.

African nations fear their farmers could plant biotech crops in their fields, contaminating native strains and jeopardising farm exports to Europe, which is resisting GM imports.

The WFP launched an appeal for $507 million in food aid, but has received only about a third so far, he said.

"I am a little bit disappointed with the pace," Morris said, adding that WFP was negotiating for another third from donors.

"I am confident we are going to get there, but the timing has been a problem because we wanted to get a significant amount of food positioned throughout the region before the October rains come," he said.

Morris said Zimbabwe, which said on Saturday it would not accept GM food aid, was still in negotiations with the WFP over ways in which it would accept U.S.-sourced maize.

He said he would meet Zimbabwean President Robert Mugabe during next week's tour to discuss the issue.