Kabul, Afghanistan (CNN) -- Two civilians were hurt when indirect fire hit Bagram Airfield in Afghanistan on Thursday, the International Security Assistance Force said.
Three rounds of fire struck the base, the force said. The civilians who were hurt were treated and released.
Officials said they do not know what type of rounds were used, although insurgents in the past have fired rockets.
Bagram is a heavily fortified base, its perimeter guarded by high fences and thick, concrete walls.
The last time the airfield came under indirect fire was on October 22. Prior to that, it was struck on September 11. In both instances, officials believe rockets were used.
CNN's Pierre Bairin contributed to this report.