It is believed they were attacked by three men, two of whom were subsequently killed by NATO forces.
The shootings come amid heightened tensions over the recent burning of Qurans that had been dumped in a garbage pit at Bagram Airfield, north of Kabul. Since then, six American troops have been killed.
The U.S. apologized for the burning, saying the Islamic texts were mistakenly sent to the garbage pit.
More than 30 Afghans have also been killed in violent riots that broke out after the incident.
The Associated Press contributed to this report.