Kabul- Drug abuse is rife in the Afghan police force with up to four out of 10 recruits testing positive for illegal drugs in some areas, a US report says.

The report for the US Congress said the illegal drugs trade "undermines virtually every aspect" of efforts to secure Afghanistan.

Afghanistan produces 90% of the world's opium and the drugs trade is a key source of funding for the insurgency.

Coalition forces are battling militants in opium-producing areas of the south.

The reliability of the poorly paid Afghan police has frequently been questioned.

The Government Accountability Office report also cited shortcomings in counter-narcotics police training which had resulted in "inconsistent" crime scene investigations and poor evidence gathering.

It said high illiteracy rates among counter-narcotics police added to the problem, as did "pervasive" drug-related corruption among local police and government officials.