The idea of turning farms into fuel plants seemed, for a time, like one of the answers to high global oil prices and supply worries. That strategy seemed to reach a high point last year when Congress mandated a fivefold increase in the use of biofuels.

But now a reaction is building against policies in the United States and Europe to promote ethanol and similar fuels, with political leaders from poor countries contending that these fuels are driving up food prices and starving poor people. Biofuels are fast becoming a new flash point in global diplomacy, putting pressure on Western politicians to reconsider their policies, even as they argue that biofuels are only one factor in the seemingly inexorable rise in food prices.

In some countries, the higher prices are leading to riots, political instability and growing worries about feeding the poorest people. Food riots contributed to the dismissal of Haiti’s prime minister last week, and leaders in some other countries are nervously trying to calm anxious consumers.

At a weekend conference in Washington, representatives of poor countries that have been hit hard by rising food prices called for urgent action to deal with the price spikes, and several of them demanded a reconsideration of biofuel policies adopted recently in the West.