Image caption Around 90% of the world's opium comes from Afghanistan

The price of Afghan opium rose dramatically in 2011, the UN has said.

Opium poppy farmers in Afghanistan probably earned more than $1.4bn (£910m) last year - equivalent to 9% of the country's GDP, it estimates.

Prices started to rise in 2010 after the poppy crop was hit by a fungal disease.

The head of the UN's Office on Drugs and Crime said opium helped fund the Taliban insurgency and fuelled corruption in Afghanistan.

"Opium is a significant part of the Afghan economy," Yury Fedotov said.

Around 90% of the world's opium comes from Afghanistan, according to the office, which carries out an annual survey of production there.

The Afghan Opium Survey for 2011 found that the value of opium in the country had increased by 133%.

Areas of poppy cultivation which had been affected by the fungal disease in 2010 recovered and yields went back up.

Last year's survey had predicted a rise in poppy planting as farmers responded to higher market prices.

Three provinces which had been declared "poppy-free" (estimated to have less than 100 hectares of opium cultivation) are now affected by poppy cultivation once again - Kapisa in the east, and Baghlan and Faryab in the north.

Opium derives from the sap produced by poppy seed heads after flowering. This can be refined into morphine - which can then be further processed to make heroin for the illegal drugs trade.