Kabul, Afghanistan (CNN) -- Three members of the Afghan National Security Forces have been killed after a helicopter was hit by munitions fired into Bagram Airfield, a NATO official said Tuesday. Four rounds landed in the airfield on Monday around 10 p.m., said Lt. Col Hagen Messer, a spokesman for NATO's International Security Assistance Force (ISAF). One of the rounds struck an ISAF helicopter, killing the three Afghan troops and wounding two ISAF service members, Messer said. ISAF is not yet able to confirm the type of the rounds or whether they were rockets or mortars, said Messer. He added that an investigation was under way to establish the details of what happened. Rewarding Afghanistan's torturers?
Two ISAF service members are wounded in the incident . A helicopter is hit after rounds are fired into the airfield . ISAF has begun an investigation into what happened .