Poor families in some countries spend as much as eighty percent of their money on food. They have watched helplessly as rising prices in recent months have created the worst food crisis in more than thirty years. The United Nations World Food Program says high food prices could push one hundred million people into hunger.

Half the world's people depend on rice as a main part of their diet. Yet the price has tripled in the past year. Inflated costs for fuel and fertilizer have played a part.

Economist Nathan Childs at the United States Agriculture Department says another reason is the falling value of the dollar. This has hurt the ability of some nations to buy food.

But he says the main reason is export limits in some rice-growing nations. This means less rice on the world market.

Nathan Childs is an expert on rice markets. He notes that the harvest for the latest growing season was the largest on record. But India, Vietnam and others have restricted exports to keep prices down at home and protect supplies.

Thailand's agriculture minister says his country will never restrict rice exports. He told the Reuters news agency Thursday that Thailand has enough supplies to meet demand at home and for export.

Thailand is the world's largest rice exporter. Recent signs of an increase in supplies have helped ease record prices for Thai rice. Prices rose last week above one thousand dollars a ton.

Vietnam, the second biggest exporter, has banned exports until June. And Vietnamese officials have now warned that non-food traders who buy rice for speculation will be severely punished.

Speculators try to predict future price movements. They take greater risks than average investors. Experts say speculation is a necessary part of market activity.

But the head of the U.N. Environment Program blames it for the high food prices. "We have enough food on this planet today to feed everyone," Achim Steiner told the Associated Press.

Earlier, a member of the United States Commodity Futures Trading Commission rejected the idea that speculators are the main cause. Bart Chilton blamed reduced harvests and grain supplies and the falling value of the dollar.

U.N. Secretary General Ban Ki-moon called heads of major international agencies to a meeting this week in the Swiss capital, Bern. He says high food prices could harm world trade, economic growth, social progress and political security.