(CBS News) WASHINGTON - Friday's attack in which an Afghan in uniform killed three American servicemen in Afghanistan brought the number of Americans killed this week by "green on blue" attacks to seven. This comes as American forces are trying to train the Afghans to take control of their own security.
Operating side-by-side with Afghan forces is the linchpin of the American strategy in Afghanistan -- giving them the support they need to take over the fighting so U.S. troops can come home.
It is a daunting task, which requires remaking a force plagued by corruption, incompetence and illiteracy.
Adding to the problem is a growing number of insider attacks in which Afghan soldiers or policemen -- or someone wearing their uniform -- turn their weapons on American soldiers, like Spc. Ethan Martin, whose body came home this week.
So far this year, there have been 25 insider attacks compared to 21 in all of 2011. The attacks this year have killed 31 members of the NATO coalition fighting in Afghanistan -- 17 of them American. Put another way, one out of every 12 Americans killed this year have died at the hands of supposed allies.
Still, Gen. John Allen, the commander in Afghanistan, insists insider attacks are rare compared to the overall number of troops in what's officially called the International Security Assistance Force -- ISAF for short.
"Every case where one of these occurs," said Allen, "that same day there are tens of thousands of interactions between the Afghans and ISAF forces that don't go that way."
There are 84,000 U.S. troops now in Afghanistan along with 40,000 from other countries operating with 332,000 Afghan army and police forces -- all trying to defeat an enemy estimated at 20,000. Those numbers will change as the U.S. draws down its forces down to 68,000 by October, but the Americans who stay will remain vulnerable to insider attacks.