At least five people have been killed and at least two injured in a suicide bomb attack on a counter-narcotics unit in the southern Afghan province of Helmand, a police official has said.

The attack took place on Thursday in Lashkar Gah, the provincial capital, Kamaludin Khan, the deputy provincial police chief, said.

Four police officers and a child were killed in the bomb blast, Khan said, adding that two police vehicles were damaged by the explosion.

Khan said that the attack was carried out by "enemies of Afghanistan," a term often used to refer to Taliban fighters.

Drugs trade

Helmand is one of Afghanistan's most productive provinces for opium, the main ingredient in heroin.

About 90 per cent of the global trade stems from Afghanistan. Up to $500m of the illegal trade goes to Taliban fighters and criminal groups, the UN estimated last year.

Geneal David McKiernan, the senior US and Nato commander in Afghanistan, has said that his troops have increased their work combatting drug operations during the past four months.

The forces are putting a particular focus on investigating drug operations that could be tied to armed opposition fighters.

McKiernan said on Monday that heroin-trafficking was "a debilitating system across this country that eats away at good governance, eats away at progress and it certainly provides a funding source for the insurgency".