(Reuters) - Leading members of the Group of 20 nations are prepared to trigger an emergency meeting to tackle soaring grain prices caused by the worst U.S. drought in half a century and poor crops from the Black Sea bread basket.

France, the United States and G20 president Mexico will hold a conference call at the end of August to discuss whether an emergency international meeting is required, aiming to avoid a repeat of the 2007/08 food price spike that triggered riots in poorer countries in 2008.

Analysts had little hope of concrete action, although there could be further calls for the United States to change its biofuels policy in response to the crisis while Russia will be encouraged not to impose an export ban.

The United States uses 40 percent of its corn crop to produce biofuel ethanol, drawing criticism from some groups which argue against the use of food for fuel when hunger is widespread in some poorer countries.

"They might talk about the U.S. ethanol mandate requirements but I don't seem them making any massive responses at the moment. They don't have a lot of tools at their disposal," said analyst Muktadir Ur Rahman Of Capital Economics.