The U.S. military has launched a criminal investigation into allegations that a small number of U.S. soldiers were responsible for the unlawful deaths of as many as three civilians in Afghanistan, the military said Thursday.

The case includes allegations of illegal drug use, assault and conspiracy, the United States Forces-Afghanistan said in a statement.

One soldier has been put in pre-trial confinement, but no charges have been put forward. The probe started earlier in May after investigators received "credible information" about the case from the soldiers' unit.

The incident comes amid dogged efforts by the U.S. military to avoid civilian casualties during war operations, such as raids and airstrikes. The public outcry against such deaths and injuries has undermined support for international and Afghan government troops.