By Adam Levine, with reporting from Tim Lister and Chris Lawrence
An Afghan soldier's killing of four French troops on Friday brought a disturbing issue to center stage in the long Asian war - attacks by local security forces against coalition troops.
"We believe that they do appear to be increasing in frequency in recent months," Pentagon spokesman John Kirby told reporters. "We've seen the numbers increase in recent months, certainly."
The incidents are a mere fraction of the total coalition deaths in the war. But they may feed a climate of uncertainty and even mutual suspicion between Afghan units and their coalition partners at a time when NATO's International Security Assistance Force is trying to hand over control of more districts and provinces to the Afghan National Army.
The latest killings, in Afghanistan's eastern Kapisa province, prompted French president Nicolas Sarkozy to suspend its training operations and combat help, saying "the French army is not in Afghanistan to be shot at by Afghan soldiers."
Afghan soldiers and ISAF troops frequently team up and most of their efforts are successful. So, Pentagon officials don't believe there's an inherent problem in working together.
At the same time, Kirby said, "we're certainly concerned about these incidents" and ISAF is reviewing what the military calls "green on blue" attacks.
Earlier this week, the Air Force released its investigation into a 2011 killing of eight American airmen and a security contractor by an Afghan air force officer.
The probe raised questions about the relations of American and other NATO troops to their Afghan counterparts.
The Afghan air force officer, Ahmed Gul, had declared his desire to kill Americans, behaved erratically at work and frequented a mosque known for its anti-American views.
The report suggested that Gul, though acting alone, was influenced by jihadist ideology and also suffering from financial strain.
The New York Times and Wall Street Journal unearthed an ISAF report that raised the alarm about "deep-seated animosity between the supposedly allied forces" and the Afghan military.
Last year, CNN's Tim Lister reported a NATO analysis that found 52 U.S. and allied soldiers killed in attacks between 2005 and June of 2011.
Combat stress was a cause in a third of those situations, according to the analysis. Perhaps more disturbingly, a quarter of the cases were instances of Taliban militants persuading the Afghan soldier to carry out the attack, according to the report.
A senior intelligence official involved in NATO's training program told CNN last June that "battlefield conditions and frequent deployments are the leading causes" of such incidents.
"When you are in the mountains for months, you've just had enough," he said.
The official said cultural differences over the handling of weapons and the attitude of Western soldiers to Afghan women can exacerbate tensions.
Coalition officers often reprimand their Afghan colleagues for lax control of weaponry, and that can breed resentment.
The already waning support for continued U.S. efforts in Afghanistan will likely be further eroded if the American public believes Afghan soldiers pose a risk to the very troops trying to help them.
A November 2011 poll by CNN and ORC found 35% of Americans support the war and 63% oppose it. More than half of those opposed to the war initially favored it back in 2001, the poll found.
If such violence increases, it could further complicate President Barack Obama's strategy to wind down the Afghanistan war. Next year, U.S. troops will begin shifting away from combat to a training and advising role. The goal is to let Afghan security forces increasingly take the lead as the U.S. looks to withdraw most of its troops by the end of 2014.
Afghanistan's growing security force has surpassed 305,000 and is headed towards 352,000 this year.
The United States is withdrawing forces through 2012 that will bring the number of American troops down to 68,000 by next year from over 100,000 after a 33,000 troop surge last year. There will also be 38,000 troops from other NATO countries.