Afghanistan's opium production declined by almost half this year due largely to a disease that damaged poppy plants, but the land area used for growing the crop remained the same, the UN's drug agency reports.

The United Nations Office on Drugs and Crime said Afghanistan's opium production in 2010 was estimated at 3600 tonnes, a 48 per cent decrease from 6900 tons in 2009 and the lowest since 2003. Opium is the main ingredient in heroin.

The drop was caused mostly by a poppy plant infection that started to appear after spring flowering and hit the southern provinces of Helmand and Kandahar especially hard, according to the summary of UNODC's annual Afghan Opium Survey released on Thursday.

The two provinces are major growing areas in southern Afghanistan and the centre of the Taliban-led insurgency.

"This is good news but there is no room for false optimism," the UNODC executive director Yury Fedotov said in a statement.