UNITED NATIONS — With memories still fresh of food riots set off by spiking prices just two years ago, agricultural experts on Friday cast a wary eye on the steep rise in the cost of wheat prompted by a Russian export ban and the questions looming over harvests in other parts of the world because of drought or flooding. Food prices rose 5 percent globally during August, according to the United Nations, spurred mostly by the higher cost of wheat, and the first signs of unrest erupted as 10 people died in Mozambique during clashes ignited partly by a 30 percent leap in the cost of bread. "You are dealing with an unstable situation," said Abdolreza Abbassian, an economist at the United Nation's Food and Agriculture Organization in Rome. "People still remember what happened a few years ago, so it is a combination of psychology and the expectation that worse may come," he added. "There are critical months ahead." The F.A.O. has called a special session of grain experts from around the world on Sept. 24 to address the supply question. Given that the fields stretching out from the Black Sea have been the main source of a huge leap in wheat trade over the past decade, the fluctuating weather patterns and unstable harvests there will have to be addressed, he said. It is an issue not limited to Russia alone. Harvest forecasts in Germany and Canada are clouded by wet weather and flooding, while crops in Argentina will suffer from drought, as could Australia's, according to agricultural experts. The bump in prices because of the uncertainty about future supplies means the poor in some areas of the world will face higher bread prices in the coming months. Food prices are still some 30 percent below the 2008 levels, Mr. Abbassian said, when a tripling in the price of rice among other staples led to food riots in about a dozen countries and helped topple at least one government.