The United Nations says the value of opium in Afghanistan soared by 133 percent in 2011 compared to the year before.

The survey released by the U.N. Office on Drugs and Crime (UNODC) said farmers' income amounted to $1.4 billion, representing nine percent of Afghanistan's Gross Domestic Product.

UNODC Executive Director Yury Fedotov said Thursday that opium is still a significant part of the Afghan economy and provides considerable funding to an insurgency, while also fueling corruption.

The survey found that almost 60 percent of Afghan opium farmers polled last year said they were primarily motivated by the high prices they get for poppy cultivation.

In 2010, a plant disease wiped out much of the opium poppy yield, driving prices of opium even higher. The U.N. says although yields returned to pre-blight levels last year, prices remain high.

Afghanistan supplied an estimated 90 percent of the world's opium, the raw ingredient for heroin. The largest areas of opium poppy cultivation remain in the country's restive south.

The U.N. survey released Thursday shows the amount of opium produced in 2011 increased by 61 percent compared to the year before, rising from 3,600 tons in 2010 to 5,800 tons last year.

The UNODC says a simultaneous drop in the price of wheat contributed to the increase in opium poppy cultivation. Gross income from opium in 2011 was 11 times higher than that earned from wheat — the biggest difference in income since 2003.