Billions have been poured into developing sugar- and grain-based ethanol and biodiesel to help wean rich economies from their addiction to carbon-belching fossil fuels, the overwhelming source of man-made global warming.

Heading the rush are the United States, Brazil and Canada, which are eagerly transforming corn, wheat, soy beans and sugar cane into cleaner-burning fuel, and the European Union (EU) is to launch its own ambitious programme.

But as soaring prices for staples bring more of the planet's most vulnerable people face-to-face with starvation, the image of biofuels has suddenly changed from climate saviour to a horribly misguided experiment.

On Friday, the head of the International Monetary Fund (IMF) said biofuels "posed a real moral problem" and called for a moratorium on using food crops to power cars, trucks and buses.

The vital problem of global warming "has to be balanced with the fact that there are people who are going to starve to death," said Dominique Strauss-Kahn.

"Producing biofuels is a crime against humanity," the UN's special rapporteur for the right to food, Jean Ziegler of Switzerland, said earlier.

Biofuels may still be in their infancy but they are growing rapidly, with annual production leaping by double-digit percentages.

In a speech on Wednesday that set down a target for reducing US carbon emissions, George W. Bush pointed to legislation requiring US producers to supply at least 36 billion gallons (136 billion litres) of renewable fuel by 2020.

In 2007, 20 percent of grain -- 81 million tonnes -- produced in the United States was used to make ethanol, according to US think tank the Earth Policy Institute, which predicts that the percentage will jump to nearly a quarter this year.

"We are looking at a five-fold increase in renewable fuel," Bush's top climate change advisor, Jim Connaughton, said in Paris on Thursday at a meeting of the world's major greenhouse-gas polluters.

But more than half of that legislatively-mandated production would come from "second-generation" biofuels made from non-food sources such as switchgrass and wood byproducts, he said.

The EU's and the Brazilian delegates in Paris contested the link between biofuels and the world food crisis.

"This is highly exaggerated," Sergio Serra, Brazil's ambassador for climate change, told AFP.

"There is no real relation of cause and effect between the expansion of the production of biofuels and the raising of food prices. At least it is not happening in Brazil."

EU Environment Commissioner Stavros Dimas said experts would report back by the end of May on how to guarantee that Europe's planned biofuel boost would not impinge on the environment or the poor.

"There are a lot of concerns about social impacts, rising food prices and environment issues, and for all those reasons we want to insist on sustainability criteria in our legislation," he said.

Defenders of biofuels say food shortfalls have multiple causes, including a growing appetite for meat among the burgeoning middle class in China and India.

On average, it takes more than four kilos (eight pounds) of grain to produce one kilo (two pounds) of pork, and two kilos (four pounds) of grain to yield a kilo (two pounds) of beef.

Climate change may well be a contributing factor.

Some scientists fear rising temperatures and shifting rainfall patterns may be worsening water scarcity in key agriculture areas such Australia's wheat belt, and rice-growing deltas may be hit by saline intrusion from rising seas.

In addition, the surging cost of oil has had an indirect impact on many poor people, adding to the pinch caused by rising food prices.

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