General Qassem Daoud Salman, from the interior ministry's anti-drugs section, and his driver were shot dead outside a restaurant 15 kilometres (nine miles) south of the Amara district in Maysan province near the border with Iran, the source said.

A third officer was wounded in the attack by unidentified militants who fled afterwards, added the source who spoke on condition of anonymity because he is not authorised to speak to the media.

Monday's killing came days after a major drugs bust in the same area, but it was not immediately clear if the two events were linked, the source added.

The interior ministry says Maysan has one of the highest rates of trafficking and consumption of drugs in Iraq.

Violence is a near-daily occurrence there, with police and judicial officials regularly targeted.

Security forces investigating the latest murder are reviewing CCTV footage in an attempt to determine the identities of the attackers, the source said.

Security forces have intensified efforts to crack down on the trade, often announcing drugs seizures and the arrest of traffickers.

Drug consumption was rare in Iraq during the reign of dictator Saddam Hussein, with both users and dealers facing the death penalty if found guilty.

But after Saddam's 2003 ouster, narcotics use boomed across Iraq, which had long been a transit route for drugs being smuggled to Europe from Afghanistan and Iran.