A suicide bomber on foot attacked a drug eradication unit in southern Afghanistan on Thursday, killing four police officers and a child while wounding seven other officers, a police official said.

The attacker struck the patrol in Lashkar Gah, the capital of Helmand province, a major drug-producing province, said Kamal Uddin, the deputy provincial police chief.

Two police vehicles and three shops were damaged in the explosion, Uddin said. Initially he said that five policemen were killed, but later said that one of those killed was a child.

Afghanistan is the world''s largest producer of opium, the main ingredient in heroin. The Afghan drug trade accounts for 90 percent of worldwide production. The U.N. estimated last year that up to $500 million from the illegal drug trade flows to Taliban fighters and criminal groups.

The top U.S. and NATO commander in Afghanistan, Gen. David McKiernan said his troops had increased targeting of drug operations by eight- or 10-fold in the past four months, specifically for drug lords or operations that could be tied to insurgents and insurgent funding.

McKiernan told newspaper executives gathered at The Associated Press annual meeting 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."