Drug abuse is a big problem in Afghanistan’s police force, with four of every 10 recruits testing positive for illicit substances in some areas, said a report by the investigative arm of the U.S. Congress.

The Government Accountability Office said the illicit drug trade posed a major challenge to the U.S.-led counter-insurgency campaign in Afghanistan, which produces 90 percent of the world’s opium.

The Obama administration has switched previous emphasis away from eradication — which was seen as alienating farmers — to going directly after traffickers and drug labs.

Financial incentives for those who eradicate crops voluntarily is another focus, as well as help for farmers who switch from planting illegal poppy to legitimate crops such as fruit or wheat.

But the GAO report, released this week, said the police force had its own problems with illegal drugs and addiction.

U.S. State Department officials cited in the report said 12 to 41 percent of police recruits in regional training centers tested positive for illicit drugs and that the percentage likely was higher as opiates leave the system quickly.

“Many recruits who tested negative for drugs have shown opium withdrawal symptoms later in their training,” said the report.

Deficiencies in counternarcotics police training resulted in “inconsistent” crime scene investigations, poor evidence gathering and weakened cases, the report said of efforts to track down drug traffickers.

High illiteracy rates among the Counter Narcotics Police of Afghanistan, a specialist force under the interior ministry, added to poorly documented drug cases.

U.S. officials found police were not generally arresting high-level traffickers. The lack of a U.S.-Afghan extradition treaty to include drug offenses removed a “valuable channel” for prosecuting high-level offenders, the report said.

Drug-related corruption was “pervasive” at the local and district levels of government and police officials and prosecutors were often easy targets for bribery because they were frequently poorly paid, it said.

The United States has allocated about $2.5 billion since 2005 to stem production, consumption and trafficking of illicit drugs in Afghanistan, where the narcotics industry equals about one-third of the legal economy and is a big source of funding for the insurgency.

Last year, most opium poppy was cultivated in the southern and western regions, which are also the most insecure areas with active insurgent elements.