Afghan police have seized 237 tonnes of hashish in what NATO-led forces and the British government called the biggest narcotics haul in history.

The cannabis product had an estimated wholesale value of $400 million (200 million pounds), a statement by the NATO-led ISAF force said on Wednesday.

It was found in a series of trenches in southern Kandahar province, near the Pakistan border, which were then set on fire. One tonne is about the weight of a small family car.

"With this single find, they (the police) have seriously crippled the Taliban's ability to purchase weapons that threaten the safety and security of the Afghan people and the region," said General David McKiernan, commander of ISAF, the International Security Assistance Force.

Afghanistan produces more than 90 percent of the world's opium and is rapidly becoming one of the largest producers of cannabis.

"This was the largest ever single find of narcotics in history," British Foreign Secretary David Miliband said in a statement.

"It reflects the efforts of the Afghan government against the drug trade, and was so large that two aircraft were brought in to destroy the underground bunker in which the hashish was being stored."

The ISAF statement said police had also seized about 5.1 tonnes of opium, with an estimated value of $30 million, and arrested 13 drug dealers in Helmand province.