MILAN (Reuters) - Rising land degradation reduces crop yields and may threaten food security of about a quarter of the world' population, the United Nations Food and Agriculture Organisation (FAO) said on Wednesday.

Food security has been highlighted in recent months as soaring crop prices resulting from poor harvests, low stocks, high fuel prices and rising demand, risks causing starvation for millions of people in the developing world.

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"An estimated 1.5 billion people, or a quarter of the world's population, depend directly on land that is being degraded," FAO said in a statement presenting a study based on data taken over a 20-year period.

Long-term land degradation has been increasing around the world and affects more than 20 percent of all cultivated areas, 30 percent of forests and 10 percent of grasslands, FAO said

Land erosion leads to reduced productivity, migration, food insecurity, damage to basic resources and ecosystems, loss of biodiversity and also contributes to increasing emission of heat-trapping gases, the Rome-based agency said.

"The loss of biomass and soil organic matter releases carbon into the atmosphere and affects the quality of soil and its ability to hold water and nutrients," said Parviz Koohafkan, director of FAO's Land and Water Division.

According to the study, land degradation is being driven mainly by poor land management.

(Reporting by Svetlana Kovalyova, Editing by Peter Blackburn)