KABUL: US and Afghan forces seized 50 tonnes of opium and 1.8 tonnes of heroin, and killed 17 Taliban insurgents in a joint operation in southern Afghanistan on Wednesday, the defence ministry said.

The drug haul, one of the biggest in recent memory, was worth five million dollars, Mohammad Zahir Azimi, ministry spokesman, told AFP.

In a five-hour operation that began at 11:00 am (0630 GMT), the soldiers dropped from helicopters into the Kajaki district of troubled Helmand province, source of most of the world's heroin, he said.

"According to our calculation, we have destroyed five million dollars' worth of drugs today," he said.

"We seized and destroyed 30 tonnes of fertiliser, 1,000 boxes of AK-47 and TK machinegun bullets and other weapons," he said, adding that a factory for making remote-controlled bombs was also destroyed.

"Seventeen Taliban were killed, three were arrested alive," he said.

"The joint forces also destroyed a heroin-making factory," he said, adding that the joint forces suffered no casualties.

He said the fertiliser was of a type used to manufacture improvised explosive devices (IEDs), the main weapon in the Taliban arsenal and causing a large proportion of deaths among forces under US and NATO command.

More than 400 foreign troops have died in Afghanistan so far this year.

A source close to the defence ministry, who spoke on condition of anonymity, said that 16 boxes of C4 explosive made in Iran had also been seized in the operation.

Helmand province is one of the main trouble spots in Afghanistan's intensifying insurgency and the heroin produced there helps fund the Taliban supply of weaponry.

Destitute Afghanistan produces around 90 percent of the world's opium, used to make heroin sold on the streets of Europe and central Asia.

Opium production in Afghanistan has fallen for a second year with poppy cultivation down 22 percent, the UN said in a report last month, while warning that drug stockpiles were a ticking bomb in the war-torn country.

UN Office on Drugs and Crime said in the report, to which the Afghan government contributed: "For the second year in a row, cultivation, production, workforce, prices, revenues, exports and its GDP share are all down, while the number of poppy-free provinces and drug seizures continue to rise."

The UN puts the potential export value of Afghan narcotics at about 3.4 billion dollars a year and Afghan officials have said drug profits provide the Taliban with as much as 100 million dollars a year.