Drought and anti-drug campaigns helped slash Afghanistan's opium poppy cultivation by 19 percent this year compared to 2007, but the country is still far and away the world's leading source of the heroin-producing crop, the U.N. said Tuesday.

Successful anti-poppy campaigns in the country's north and east were mainly to thank for the drop in production but fields in the south — where the Taliban insurgency is strongest — remain awash in poppies, which provide the main ingredient for heroin, a U.N. report said.

And because of a rise in yield, opium production this year will fall only 6 percent compared with last year's record haul and the Taliban stand to again earn tens of millions of dollars from the drug trade.

Still, the U.N. and other drug officials said this year's results provide reason to be cautiously optimistic.

"The opium flood waters in Afghanistan have started to recede," the United Nations Office on Drugs and Crime said in its report, Afghanistan Opium Survey 2008.

Last year opium farmers cultivated 476,903 acres; this year they cultivated 388,000 acres.

Helmand province — the country's most violent region — accounted for some 66 percent of the crop by itself. Farmers there cultivated 254,513 acres, up 1 percent from last year. If Helmand province alone were a country, it would be the world's largest producer of opium.

The number of opium-free provinces rose from 13 to 18 after extensive anti-drug efforts in the north and east. Anti-drug officials were particularly proud that Nangarhar — last year's second highest-producing opium province — was free of the drug this year.

"This is a remarkable accomplishment, the first time it happens in the country's modern history," the drug report said.

The UNODC said the country's anti-poppy successes could be attributed to good local governance and drought, which hurt poppy growth in the north and east.

The U.N. also warned that the government could not let cannabis — the precursor to marijuana that is growing in popularity here among farmers — replace opium. It also said the country needs a better justice system to prosecute traffickers, landowners and corrupt government officials who turn a blind eye to the drug trade.

"Until they all face the full force of the law, the opium economy will continue to prosper with impunity, and the Taliban will continue to profit from it," the UNODC said.

It also said the world has a surplus of opium hidden in storage.

The price of fresh opium at harvest dropped 20 percent, from $86 per kilogram to $70, the UNODC said. The value at harvest of all of Afghanistan's opium crop dropped from $1 billion in 2007 to $732 million this year. The overall value of the crop rises two to four times once it hits the streets from Iran and Pakistan to Europe.

Potential opium production from this year's crop is 8,487 tons, down from 9,038 tons in 2007.

Eradication — where Afghan anti-drug police slash opium crops — dropped to 13,590 acres this year. Afghan officials eradicated 46,949 acres last year.