The US has been warned by some of the world\'s leading aid agencies that its plan to eradicate Afghanistan\'s opium fields could backfire. In a letter to new US Secretary of State Condoleezza Rice, they warn that any \"premature\" act risks destabilising large parts of the country. They call for a greater emphasis on providing rural development and alternative crops for opium farmers. The UN says drug exports now account for 60% of Afghanistan\'s economy. The US has strongly backed Afghan President Hamid Karzai\'s declared \"holy war\" against the drugs trade. Washington announced in November it would spend $780m in an attempt to eradicate the country\'s crop, which now supplies about 75% of the world\'s opium. But 20 non-government agencies - including Care, the International Crisis Group and Oxfam - have urged caution. They point out that many farmers depend on the crop to earn their living and any \"premature effort\" to wipe it out will play into the hands of government opponents. \"It\'s a policy that needs to be handled in a very careful, progressive fashion - it needs to allow for the rise of alternative livelihoods before you start eradicating the drugs,\" said Robert Templer, the Crisis Group\'s Asia programme director. They say any short-term solution could just drive opium production underground or into more remote areas. Longer-term solutions are needed, they say. This includes greater rural development, the provision of alternative livelihoods, access to infrastructure and new ways for the communities to police themselves. The US has postponed plans to spray Afghan opium crops from the air, but says it will continue to destroy them on the ground. The move came after the Afghan government reacted angrily at the mystery spraying of herbicide on opium crops in eastern areas in November. (BBC)