Afghanistan's opium harvest fell in 2008 after international efforts to persuade farmers to switch crops but was still the second biggest on record, a United Nations body said on Thursday.

While the area under cultivation was reduced by a fifth, better yields meant production dropped only 6 percent to 7,700 tons, after a record 8,200 tons in 2007, the U.N.'s International Narcotics Control Board said in its annual report.

More than seven years after the U.S.-led invasion, Afghanistan still grows more than 90 percent of the world's illegal opium poppies, the source of heroin.

Five more of the country's 34 provinces ended opium farming in 2008, taking the opium-free total to 18. Output is focused on seven provinces in southern Afghanistan where U.S. and NATO troops are fighting an increasingly bold Taliban insurgency.

"Insecurity and drug production and trafficking...are very much inter-related," INCB President Hamid Ghodse told a news conference in London. "It is very difficult to say which is the cause and which is the effect."

Poor security and corruption are the biggest obstacles in the fight against the Afghan drugs trade, the report said.

The Taliban earns $200-300 million each year from levies on the drugs trade in Afghanistan, it said, citing figures from the U.N.'s drugs and crime body.

On Wednesday, U.S. President Barack Obama said 17,000 more U.S. troops will be sent to Afghanistan [Story ID:nISL425666] to tackle the "deteriorating situation".

NATO forces are not allowed to eradicate crops although NATO allies agreed in October to allow their soldiers to carry out direct attacks on Afghan drug lords and laboratories.

Afghan officials let drug traffickers operate with impunity and those who do target the opium trade risk their lives, the report said. Last year, 78 officials trying to eradicate opium crops were killed, six times the toll in 2007.

"Corruption among officials at almost every level of the government is a major factor of the drug problem," Ghodse said.

The report also said the Afghan government was ignoring a worrying rise in cannabis production as farmers in some parts of the country switch from growing opium.

"Cannabis cultivation is becoming increasingly lucrative," the report said. "No action has been taken by the government to prevent such cultivation."

The U.N. report called on the international community to give the Afghan government more help and money to fight the drugs trade. Crop substitution must be backed by improvements to the health service, education and infrastructure, it said.

VIDEO: Afghan drugs bust by UK troops