While the motive for the attack is unknown, the incident is a rare instance of premeditated violence against addicts in the capital of the country, which produces most of the world's opium supply.



The men lay asleep in an open area when they were set upon by an unidentified gunman.



Forensic examination later showed that they were drug users.



"The shooting took place at the side of Qrough Mountain," a spokesman for Kabul police, Ferdaus Faramarz, told Reuters news agency.



According to a recent national survey, between 2.9 to 3.6 million Afghan citizens are drug users, with the vast majority addicted to heroin made from opium poppies grown in the country.



At least 20,000 drug users remain homeless, with half that number living on Kabul's city streets.

"It's a social crisis," said Dr Shokoor Haidari, deputy of the ministry's counter-drug department.

The ministry can treat no more than 40,000 people a year but far more seek help, said Haidari.



A combination of poor social services, rampant unemployment levels and ease of access to drugs all fuel the rising levels of drug abuse in Afghanistan, Haidari added.



According to the ministry of public health, at least 50 homeless drug users were killed as a result of harsh, freezing weather which has gripped most of the country over the past two months.



While the US has spent some $8.9bn since 2002 in efforts to combat narcotic production and trafficking, compelling economic incentives and political protection are pull-factors for those in the drug trade, from cultivators to producers and distributors.



Officials say there is little they can do to stop a problem, one which is entrenched in state institutions.



This month, the Afghan interior ministry announced the arrest of five high-ranking police officers, including the head of Kabul's counter-narcotics force, for suspect involvment in drug trafficking.



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