PASCAL GUYOT/AFP/Getty Images Afghan soldiers fire their machine guns under the watchful eye of their NATO trainer

Pentagon officials won’t concede it, but privately defense officials say the spate of “green-on-blue” killings by Afghans of their purported U.S. allies over the past two weeks could lead to major changes in U.S. strategy. If they continue, the U.S. will face a brutal choice: pull out, or double down.

“Let me be very clear,” Defense Secretary Leon Panetta told students Thursday night at the University of Louisville. “The brutal attacks that we have seen over the last few days on our troops will not change and will not alter our commitment to get this job done.” Pentagon spokesman George Little said earlier in the day that scrutiny of Afghan army and police recruits will be stepped up, but that the bottom line remains constant: “Our strategy of working closely with [Afghan forces] is not changing.”

Over the past 11 days – ever since U.S. troops allegedly mistakenly tossed copies of the Koran into a burn pit at Bagram, the major U.S. base in Afghanistan — six Americans have been killed by their purported allies. Two were killed Thursday in southern Afghanistan, and a pair was slain last Saturday inside the Afghan interior ministry. “Always maintain situational awareness in order to keep yourself safe,” a 2011 Afghan Ministry of Interior Advisor Guide said. “And, be vigilant – always.”

When you speak to U.S. Army and Marine officers, there’s a nervous titter in their voice as they dismiss the significance of what has happened. With fingers crossed, they are hoping – and they probably are right – that the spasm of killings has peaked and is now on the decline.

But the Taliban know that one way to drive the Americans out is to make such killings a regular occurrence. If they’ve got more sleeper agents in reserve, they’ll be rolling them out in the coming days. The Pentagon recently released data showing that 75% of the more than 45 so-called “green-on-blue” attacks since 2007 have happened in the last two years.

“The risk is so high that we may discover it through hard lessons — a.k.a. lives of senior officers and NCOs who would run the [U.S.] Army if they are not killed by the people they are advising,” someone from Afghanistan anonymously posts on Best Defense. “No one wants to talk about the big elephant in the room: How many infiltrators or complicit Taliban really are in the ANSF [Afghan National Security Forces]. Is it really worth the risk to put leaders our there like this?”

That’s an amazing question. If such murders subside, the Obama Administration will be able to cling to its all-out by 2015 strategy. But that’s the essential conundrum: for the U.S. troops to leave, they have to be able to train Afghan security forces to take their place. But if those trainees keep killing their trainers, the chances of success for such a program plummet.

“The apparent randomness and unpredictability of the murders of the six U.S. soldiers creates the terror effect that destroys trust,” ex-DIA analyst John McCreary writes on his NightWatch blog . “All joint patrols with Afghans carry an increased probability of murder that overrides can-do attitudes and positive thinking.”

But for now, Pentagon spokesman Little says U.S. troops will “stay the course” in Afghanistan. “Our mission is one of transition, and it’s working,” he said. “We have over 300,000 [Afghan security forces] right now who are working alongside ISAF [International Security Assistance Force] personnel to help secure their own country.”

It’s too bad neither Little – nor anyone else on the allied side – knows how many, if any, of those 300,000 are Taliban agents patiently waiting word to kill their American mentors. That has U.S. troops in Afghanistan twitchy and concerned.