The military has launched an investigation into the incident, which appeared to stem from confusion on the battlefield in the southern province of Helmand, Afghanistan, US officials said.
Fighter jets and combat helicopters are usually called in to provide close air support for coalition troops pinned down by insurgent fire, while drones tend to be used for manhunts targeting Taliban figures.
The strike claimed the lives of Navy Seaman Benjamin Rast, 23, of Niles, Michigan, and Staff Sergeant Jeremy Smith, 26, of Arlington, Texas, officials said.
NBC News, which first reported the incident, said that the two service members were part of a unit ordered in to reinforce Marines coming under heavy fire from insurgents outside Sangin, the scene of fierce fighting for years.
The Marines near Sangin, watching a video feed from the armed Predator drone overhead, saw infrared images moving towards them and may have concluded those "hot spots" were insurgents instead of fellow Marines, NBC reporte