Sixteen Afghan police officers were killed in an airstrike in Helmand province on Friday, a provincial government spokesman said on Saturday.
The friendly-fire deaths occurred during a US-supported Afghan National Defense and Security Forces operation targeting militants in Gereshk district.
"Aerial fires resulted in the deaths of the friendly Afghan forces who were gathered in a compound," US Forces-Afghanistan said.
Two other officers were injured in the incident, Omar Zwak, a provincial government spokesman, said.
Helmand, a sprawling province in southern Afghanistan, has been the scene of much combat in the long Afghan war.
"We would like to express our deepest condolences to the families affected by this unfortunate incident," US forces said in a statement. "We have notified Afghan officials of this incident. An investigation will be conducted to determine the specific circumstances that led to this incident."
Friendly fire attacks, when forces are accidentally struck by their own side, and insider attacks, when members of Afghan security forces deliberately target the troops, have occurred on occasion during the long Afghan conflict.
Militant groups such as the Taliban, the Haqqani network and ISIS, foes of the Afghan forces, have posed a stiff challenge in the war.
Afghan and US forces launched an offensive against ISIS in early March. Gen. John Nicholson, commander of US and NATO troops in Afghanistan, has pledged to drive the terror group out of the country by the end of 2017.
Unlike elsewhere in Afghanistan, where US troops primarily serve a training and supporting role, US Special Operations Forces have been directly engaged in the offensive against ISIS in Afghanistan, conducting raids and strikes in the country's eastern provinces.