Bans on food exports by countries hit by rising prices are impeding efforts to get sustenance to some of the world's most needy people, the head of the World Food Programme said.

Executive director Josette Sheeran appealed to countries to exempt humanitarian agencies from their export restrictions or taxes so that states with excess food stores could contribute to feeding the world's hungry.

"As countries deal with the food crisis, more and more are shutting down their markets for export ... It is becoming more and more difficult to buy," she said, speaking on the sidelines of an African summit in Egypt on Tuesday night.

"So we're really issuing a call to all governments to exempt humanitarian purchases from export bans and from extraordinary tariffs," she said.

Experts say poor harvests, high fuel costs and rising demand, especially from fast-growing Asian nations, mean one billion people worldwide are now threatened by hunger. Some countries have restricted exports of food products in response to rising prices.

Africa's top diplomat, Jean Ping, said on Friday that African states must unite to reduce the impact of soaring food prices that have hit their citizens harder than the rest of the world.

"We are procuring globally, but for us it is not whether or not the restaurants will be stocked, but it is whether or not the most vulnerable people will receive the food that they urgently need," Sheeran said.

Japanese media have said that a statement on rising food prices to be issued by Group of Eight leaders this month will stress the importance of self-restraint in curbing exports.

Sheeran said 80 percent of WFP funds were now spent buying food in the developing world to feed the hungry in poor countries, and said she considered that "a revolution in food aid that is very positive".

But the World Food Programme was now often having to tender for food globally rather than purchasing from the same regions where food was needed, meaning that it had to bring stocks in from much farther away.

"I have food in Burkina Faso that I can't export that's needed in Niger. So our local purchase is very helpful to local economies but if they feel they have shortages, they are putting bans on," Sheeran said.

She said that about 20 countries that she described as "critical markets" for the WFP had put export bans in place, at least for some foods. Others have imposed tariffs.

"We don't want to buy from countries that don't have enough food to feed their own people," she said. "So we are really obviously sensitive to the need to look where there are surpluses." - Reuters