Afghan authorities say militants attacked a police post and killed six officers in southern Helmand province Sunday.

The Afghan Defense Ministry also says a roadside bomb in neighboring Zabul province killed an Afghan army soldier and wounded three others Sunday.

Afghan and international forces have been fighting Taliban militants since 2001. U.S. President Barack Obama is sending an extra 21,000 troops to Afghanistan to try to crush the insurgency.

The U.S. military said Sunday it is investigating a shooting incident involving four American contractors from a controversial private security firm formerly called Blackwater.

The military says the May 5 shooting caused the death of one Afghan, and wounded two others. A military statement released after the incident said the contractors fired at the victims' vehicle because the believed it to be a threat. 

The security company, now known as Xe, says it terminated the contracts of the individuals involved in the shooting, and ordered them not to leave Afghanistan without the approval of the U.S. Department of Defense.

The contractors' lawyer, Daniel Callahan, says his clients are being held against their will.

The security company is well-known for its presence in Iraq, where private security forces were involved in a 2007 shooting that killed at least 17 Iraqi civilians.

Some information for this report was provided by AFP, AP and Reuters.