"I'm mad as hell about them, to be honest with you," Gen. John Allen said in response to a question about the incidents, which the military calls insider, or green-on-blue, attacks. "We're going to get after this. It reverberates everywhere, across the United States. You know, we're willing to sacrifice a lot for this campaign. But we're not willing to be murdered for it."
President Barack Obama's plan for withdrawal from Afghanistan calls for the U.S. to train Afghan security forces to defend their own country as most of the remaining 68,000 U.S. troops return home in 2014.
In the meantime, Allen -- who said that the "vast majority" of Afghans and Afghan soldiers stood with the U.S. -- warned that Americans should be prepared to see the deadly attacks continue.
"The enemy recognizes this is a vulnerability," Allen said. "You know, in Iraq, the signature weapon system that we hadn't seen before was the IED. We had to adjust to that. Here, I think the signature attack that we're beginning to see is going to be the insider attack."
Allen's comments come at a moment when the fate of the troop-training program in Afghanistan is seriously in doubt.
U.S forces resumed joint patrols with their Afghan counterparts, halted after a spikein insider attacks prompted a temporary reevaluation period, only last week.
But in the days since the patrols were resumed, several more suspected insider killings have occurred, including a wild firefight over the weekend between American and Afghan units that left two Americans and five Afghan soldiers dead. The military has called the incident the result of a miscommunication.
On Sunday, the Pentagon formally announced the death of the 2,000th servicemember in the war in Afghanistan, a gruesome benchmark in a conflict that has now lasted 11 years. So far this year, about 1 in 5 combat-related deaths of a U.S. or NATO-allied troop has been at the hands of Afghan friendly fire, according to Reuters.
Last week, a former top defense official who currently advises the campaign of President Barack Obama said that the green-on-blue killings were largely a sign of "Taliban desperation," and a "very occasional" occurence.