"Regaining lost land is too expensive. Prevention is the only solution for countries that do not have enough resources," said Hama Araba Diallo, executive secretary of the United Nations Convention to Combat Desertification.

Israel was one example of a country that had managed to regain land lost to spreading desert but at a high technological cost, the UN expert said.

"For farmers in Mali or Mongolia, we can only say 'please protect that topsoil from washing away'," he told reporters at the Foreign Correspondents' Club of Japan, adding that thousands of years were needed for topsoil to recover to a state where it could yield crops.

Land degradation causes crop losses of around 42 billion dollars a year, according to the UN Environmental Programme (UNEP), which has declared 2006 a year of focus on deserts and desertification.

The UN estimates that about 27 percent of China is now desert and economic losses from growing dust bowls there amount to 6.5 billion dollars a year. Central Asian countries are also affected by land degradation.

Countries, especially developing countries, must integrate more desertification prevention measures into their economic policies to tackle the effects on agriculture, the economy, health and society, Diallo said.

Deserts cover 41 percent of the world's surface and desertification menaces about 250 million people on five continents. Some 1.2 billion people in the world's 110 poorest states are under threat, according to the UN.

The main causes are believed to be over-harvesting, cattle-breeding and overgrazing, deforestation and climate change.

The most endangered region is Africa, especially in the south and in the Sahel countries bordering the Sahara Desert, followed by Central Asia and China, UN experts warn.

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