Josette Sheeran said the U.N.'s hunger-relief agency will need to raise $500 million more than it expected to carry out scheduled relief operations in 2008, following a 70 percent spike in food prices since 2002.

The WFP already is under the gun, seeking to mobilise donors to raise $2.9 billion to feed people this year.

"In some cases, we will see these programmes unable to stay whole unless we are able to get additional resources at some time in the near future," Sheeran said, adding new cash would have to start arriving by May or June.

She added that without the additional contributions "we would be able to reach less people".

Beyond the extra $500 million for already scheduled programmes, more cash is needed to help populations hit by unforeseen crises, she said.

Sheeran pointed to the example of Afghanistan, where 2.5 million people need relief after wheat shortages. The estimated cost for the Afghanistan aid was $77 million, she said.

"We are seeing additional needs that had not been identified even as of six months ago," she said.

Growing demand for food in China and India, a shift toward more meat-oriented diets and the use of crops for biofuels have all helped drive up the cost of food. The U.N. Food and Agriculture Organisation (FAO) has warned that record food prices are unlikely to ease in the foreseeable future.

Although the WFP has sought to limit costs by purchasing food in local markets, higher prices worldwide have meant that its budget to relieve hunger has been stretched thin.

"We're able to actually get less food in those pipelines now (for planned programmes) than we were three months ago, or six months ago, or nine months ago," Sheeran said.

The WFP is holding talks about the future of food prices with range of experts from organisations including the FAO, Oxfam, and Michigan State University.

Sheeran, a former undersecretary for economic affairs at the U.S. State Department who took over at the WFP last April, said donors were "absorbing" the information. The United States is the WFP's top donor.

"This is really a new dynamic in global food markets that is affecting the most vulnerable and we are all coming to grips with how big the challenge is," she said, when asked how donors were responding.