"We continue to be deeply concerned about the insider threat," Kirby told reporters at the Pentagon on Thursday. 

The insider attacks have undermined the trust between allies and efforts to prepare Afghan troops to take over their own security as international combat troops prepare to withdraw. 

In the latest incident, a witness said Afghan civilians were talking to the soldiers outside their base when a man in an Afghan army uniform opened fire on them with a machine gun. 

He added that the wounded soldiers were evacuated by helicopter, while the others "took us aside in fear of a possible gun battle." Eman, who gave only one name, said the Afghan who opened fire escaped toward some trees and into a nearby village. 

Wardak, located close to Kabul, is considered a Taliban hotbed and has been the scene of heavy fighting over the past year. 

On last year's anniversary of the Sept.