Afghanistan, the world’s biggest producer of opium, has also become a major source for cannabis, overtaking Morocco as the top producer of hashish, the UN's Office on Drugs and Crime said Wednesday.

"While other countries have even larger cannabis cultivation, the astonishing yield of the Afghan cannabis crop makes Afghanistan the world’s biggest producer of hashish," UNODC Executive Director Antonio Maria Costa said in a statement.

An UNODC survey estimated Afghanistan farmers produce 145 kilograms per hectare of hashish, the resin produced from cannabis, as compared to around 40 kg/ha in Morocco, for an overall amount of 1,500 to 3,500 tonnes a year.

It estimated that 10,000 to 24,000 hectares of cannabis are grown in Afghanistan every year worth 39 to 94 million dollars, which is still only 10 to 20 percent of the value of opium production to farmers.

Afghanistan's illicit drugs industry is worth up to three billion dollars a year, controlled by militants and gangs who use cross-border routes to smuggle drugs to Pakistan and Iran, and bring arms and fighters back in.

Cannabis brought farmers a higher estimated net income due to lower cultivation costs, some 3,341 dollars per hectare compared to 2,005 dollars for opium, but its need for irrigation and shorter shelf life limits its overall attractiveness, the UNDOC survey found.

It found that like opium, cannabis was mostly being grown in areas of instability in the south of the country, being exported, and was a source of financing for insurgents fighting the Afghan government and foreign troops.

"All drugs in Afghanistan, whether opium or cannabis, are taxed by those who control the territory, providing an additional source of revenue for insurgents," said Costa.

The solution to the opium and cannabis problems is the same, he said.

"By improving security and development in Afghanistan’s drug-producing regions, we can knock out the world’s biggest supplies of both hash and heroin," said Costa.

Last year 6,900 tonnes of opium were produced in Afghanistan, or more than 90 percent of the world total, according to UNODC estimates.

The Afghanistan government launched at the beginning of March a campaign to eradicate poppy fields with aid to farmers to cultivate alternative crops.