Opium production and trafficking account for one-third of Afghanistan's economy and is complicating U.S. efforts to rebuild the country and its government, a State Department report said Wednesday.

The number of acres under opium cultivation dropped 48 percent last year, but output declined by only 10 percent because of good weather, the report said. Opium is the main ingredient of heroin.

"Afghanistan's huge drug trade severely impacts efforts to rebuild the economy, develop a strong democratic government based on rule of law, and threatens regional stability," the report said.

It said dangerous security conditions and corruption are hindering government and international efforts to combat the drug trade and provide alternative incomes.

The reduction in planting, the report said, may be credited to several factors, including a surplus crop from 2004 and public information efforts against poppy cultivation.

The annual report, mandated by Congress, was released shortly after President Bush made an unannounced four-hour visit to Afghanistan while en route to India.

The 900-page study, titled "International Narcotics Control Strategy Report," examines production, trafficking, money laundering and financial crimes in all countries.

The report offered a generally upbeat assessment of the situation in Colombia, the world's leading producer of cocaine. Colombia had a record year in 2005 for drug eradication and interdiction, and in the extradition of suspected traffickers to the United States, the report said.

Even so, the report said the Colombian government detected "massive replanting and reconstitution efforts" by traffickers in some areas.

Cocaine is derived from coca, which is cultivated largely in areas under the control of illegal armed groups that earn substantial sums from trafficking in cocaine and heroin.

In some other countries, the report said:

_Coca cultivation has increased in Peru and Bolivia.

_Opium poppy production in Pakistan decreased by 58 percent last year. But Pakistan "is on the frontline of the war against drugs as a major transit country for opiates and hashish from neighboring Afghanistan."

_There was no change in Mexico's role as the principal transit country for cocaine entering the United States. Mexico also served as the main foreign source of marijuana consumed in the U.S. as well as a major supplier of heroin.

_Venezuela is a key transit point for drugs leaving Colombia, a situation aggravated by "rampant corruption at the highest levels of law enforcement and a weak judicial system."

"As a result, organized crime flourishes, with seizures and arrests of underlings more an annoyance than a threat," the report stated. The Bush administration has clashed repeatedly in recent months with Venezuelan President Hugo Chavez.

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