Profits from opium cultivation are fuelling the insurgency in Afghanistan, the United Nations said on Friday, in a new call on NATO to crack down on the country's burgeoning drugs trade.

Releasing the final draft of its 2007 Afghan opium survey which in August already showed a surge in production, the U.N. Office on Drugs and Crime (UNODC) said the total export value of opiates in Afghanistan stood at about $4 billion.

Farmers saw around a quarter of that total, while district officials took a percentage through a levy on the crops. The rest was shared among insurgents, warlords and drugs traffickers, it said.

The potential windfall for criminals, insurgents and terrorists is staggering and runs into the hundreds of millions of dollars, UNODC Executive Director Antonio Maria Costa said in a statement released in Brussels.

Since drugs are funding the insurgency, NATO has a self-interest in supporting Afghan forces in destroying drugs labs, markets and convoys. Destroy the drug trade and you cut off the Taliban's main funding source.

Costa's statement made no reference to the use of aerial spraying of poppy fields, an eradication method backed by the United States but opposed by President Hamid Karzai and a number of NATO nations on grounds including its ecological impact.

The 26-nation alliance has in the past stressed its 40,000 forces have no mandate to lead eradication of the poppy trade, but has said it was looking to do more within its existing remit to support counter-narcotics efforts by Afghan authorities.

The UNODC report confirmed its August estimate that Afghanistan's potential for opium production leapt a third to 8,200 tonnes this year, with the area under poppy cultivation rising 17 percent to 193,000 hectares.

The figures show Afghanistan now accounts for 93 percent of world opium production and is the biggest narcotics producer since 19th century China, highlighting the failure of Afghan and British-led international efforts to tackle the problem.

UNODC stressed its estimate of the value of the opium trade in a country with a total licit national output of just $7 billion was based on local prices.

The wholesale price of a gram of heroin grew with every border crossed, it noted, rising from $2.50 in Afghanistan itself to $3.50 in Pakistan and Iran, $8 in Turkey, $22 in Germany, $30 in Britain and $33 in Russia, it said.

Afghanistan has suffered two years of rising violence as the Taliban, ousted from power by U.S.-led forces in 2001, have relaunched a campaign to overthrow Karzai's pro-Western government and eject international troops.

Afghanistan