KABUL (Reuters) - Gunmen shot dead nine homeless drug users in the Afghan capital, officials said on Sunday, shining a light on chronic drug abuse in the world’s biggest producer of opium but a rare incident of apparently coordinated violence against addicts.

The motive for the Saturday night attack by the unidentified gunmen in Kabul was not known and police said they were investigating. The men had been sleeping in an open area and a forensic examination had shown they were drug users.

“The shooting took place at the side of the Qrough mountain,” a spokesman for Kabul police, Ferdaus Faramarz, told Reuters.

There are an estimated 2.5 million drug users in Afghanistan, the Ministry of Public Health says, with most thought to addicted to heroin made from opium poppies grown in Afghanistan.

Some 20,000 drug users are homeless, with half that number in Kabul, at times straining relations with residents of some communities.

“It’s a social crisis,” said Dr Shokoor Haidari, deputy of the ministry’s counter drugs department.

The ministry can only treat 40,000 people a year but far more seek help, said Haidari.

Lack of social services, unemployment and easy access to drugs have fueled drug abuse in Afghanistan, Haidari said.

Harsh winter weather killed at least 50 homeless drug users in the past two months, the Ministry of Public Health said.

Afghanistan has been the world’s biggest producer of opium for years despite some $8.9 billion spent since 2002 by the U.S. government to stop production and trafficking in narcotics.

With compelling economic incentives and politically protected networks - from cultivators to producers and distributors - deeply entrenched, officials say there is little they can do to stop it.

The Interior Ministry this month announced the arrest of five top police officials, including the head of Kabul’s counter-narcotics force, for suspected involvement in drug trafficking.