World leaders open a conference on the global food crisis on Tuesday, with human rights activists and the World Bank demanding action to curb soaring prices that are pushing an estimated 100 million people into hunger.

The United Nations' Food and Agriculture Organisation (FAO) initially called the summit at the end of last year to discuss the risks posed to food security by climate change.

But soaring food prices have shifted the focus of the Rome summit.

The cost of major food commodities has doubled over the last couple of years, with rice, corn and wheat at record highs. Some prices have hit their highest levels in 30 years in real terms - provoking protests and riots in some developing countries, where people may spend more than half their income on food.

Delegates will discuss a range of issues such as aid, trade and technology to improve farm yields, but hunger campaigners have singled out biofuels - often made by converting food crops into fuel - as a prime culprit.

"Countries are justifying the pursuit of biofuels on the grounds that they offer a means to reduce emissions from transport and improve energy security," the campaign group Oxfam said in a report issued on Tuesday.

"But there is mounting scientific evidence that biofuel mandates (policy support) are actually accelerating climate change by driving the expansion of agriculture into critical habitats such as forests and wetlands."

Even though the United States is channelling about a quarter of its maize crop into ethanol production by 2022, and the European Unions plans to get 10 percent of auto fuel from bio-energy by 2020, biofuel supporters say the effect on global food prices is small.

U.S. Agriculture Secretary Ed Shafer said biofuels accounted for only around 3 percent of the total food price rise. Oxfam says the real impact is about 30 percent.

World Bank chief Robert Zoellick said the issue should not be allowed to dominate the summit, although biofuels clearly competed with food production. However, he said Africa could benefit from sugar-based biofuel production, as Brazil has.

The World Bank estimates that higher food prices are pushing 30 million Africans into poverty. Zoellick said African leaders wanted action, not words.

"It would be unfortunate if (bio-energy) becomes the sole point of debate, because then we would not meet what poor countries tell me they want, which is resources for safety net programmes, seeds and fertilisers, and export bans lifted," he told Reuters.

Brazil, a pioneer in sugar-cane based biofuels, is set to defend them at the summit. Its foreign minister, Celso Amorim, said fair trade and the abolition of rich countries' subsidies to farmers were crucial issues for the summit.