Head of UN office on drugs and crime Antonio Maria Costa says Afghan gangs and Taliban stockpiling drug to manipulate street price in the west

The United Nations' drugs czar told Nato that Afghan insurgents were withholding thousands of tonnes of heroin and treating their drugs like "savings accounts" to manipulate street prices in the west, according to a leaked US cable.

Antonio Maria Costa, head of the UN's office on drugs and crime, told Nato representatives that the Taliban and organised crime gangs had withheld 12,400 tonnes of opium from the international market to keep the price of heroin and opium at a profitable level. The opium allegedly withheld by insurgents was worth around $1.25bn (£800m). Each tonne of opium is said to be worth around $100,000 and can be used to produce 100kg of heroin.

The US cable appears to show that the UN believed that the Taliban and other insurgents in Afghanistan were well-organised, aware of the market and focused on maintaining a viable price for the drug.

Reports on the Taliban's involvement in the drug trade from 2009 have focused on the splits between different drug gangs in Afghanistan and highlighted their fierce rivalries.

Costa's claims, reported in a confidential document, were expressed at a meeting on 18 September 2009. He was briefing Nato and its partners on the results of the 2009 Afghanistan Survey, the UN's annual assessment of the drugs industry in the country.

Afghanistan is the world's biggest exporter of heroin and opium most of which is grown in Helmand province, where British troops continue to lose their lives.

Under the heading "Opium Stocks Remain High", the cable states: "Costa said that Afghanistan has 12,400 tonnes of opium stocks because it produces more than the world consumes. Costa believes that the insurgency is withholding these stocks from the market and treating them like 'savings accounts'. He said the stocks pose a serious threat as it could be used to finance the insurgency. Costa encourage intelligence organizations to to keep focus on the storage and movement of Afghanistan's opium stocks."

Costa's reported opinion was not part of the UN's final 2009 Afghanistan Survey.

According to the cable, opium trends were positive overall and showcased a major decline in opium cultivation – by 22% in 2009, the lowest in 15 years.

"Costa said that even though Afghanistan was among the most impoverished countries in the world poverty was not the main factor. Costa said abandoning opium cultivation dies not produce humanitarian crisis. He said market forces caused a shift in opium prices and could easily influence farmers to grow ilicit crops if high market prices and revenue could be gained from them," the cable said.

Costa retired from his UN position earlier this year. He prompted an international debate when he claimed billions of pounds of laundered drug money from organised crime had been used to prop up many of the world's major financial institutions during the financial downturn.

According to the UN's 2010 Afghanistan survey, the total 2010 opium production is estimated at 3,600 metric tonnes, down 48% from 2009. The decrease was largely due to a plant infection hitting the major poppy-growing provinces of Helmand and Kandahar particularly hard. As a result of the damage, yield fell to 29.2kg per hectare, from 56.1kg per hectare compared with the previous year.

This year's survey has acknowledged that Afghan drug lords have previously stockpiled some drugs. About 87% of total opium production took place in the south and 12% in the west in 2009.

A spokesman for UNODC declined to comment.