At the same time, prices set in international rice trading now have an increasingly important effect on prices within countries. This has been particularly true in an age of Internet and mobile phone communications when even farmers in remote areas can learn about distant prices and decide whether their own buyers are giving them a fair price.

Even before governments imposed restrictions this week, trading companies in exporting nations had become increasingly reluctant to sign contracts for future delivery as they wait to see how high prices will go.

“The market has pretty much ground to a halt for the past few weeks,” said Ben Savage, the managing director for rice at Jackson Son & Company, a commodities trading firm in London. Soaring prices are already causing hardship across the developing world.

In a crumbling covered market in an old neighborhood of Hanoi, Cao Minh Huong, a ceramics saleswoman, said that rising food prices, especially for rice, were forcing her to change her diet. “I’m spending the same amount on food but I’m getting less,” she said.

Together with rising prices for other foods, like wheat, soybeans, pork and cooking oil, higher rice prices are also contributing to inflation in many developing countries. Retail rice prices have already jumped by as much as 60 percent in recent months in Vietnam, trailing increases in wholesale prices but leading a broader acceleration in inflation. Prime Minister Nguyen Tan Dung of Vietnam announced Wednesday that the government’s top priority now was fighting inflation. Overall consumer prices are more than 19 percent higher this month than last March. . The inflation rate has nearly tripled in the last year.

Rice is unusual among major agricultural commodities in that most of the major rice-consuming countries are self-sufficient or nearly so. Only 7 percent of the world’s rice production is traded across international borders each year, according to figures from the United Nations Food and Agriculture Organization in Rome.

Nguyen Van Bo, the president of the Vietnam Academy of Agricultural Sciences, which oversees government farm research institutes, said in an interview that the government expected rice production to rise further by 2010 despite the rapid expansion of residential housing and factories into what had been prime rice-growing land. But the government needs to train farmers to alternate corn with rice to defeat rice pests like the virus, he said.