The top U.S. commander in Afghanistan, Marine Corps Gen. John Allen, said Thursday that up to 25 percent of fatal attacks on Americans there may be the result of Taliban coercion or infiltration of Afghan security forces.

Earlier this month, NATO officials attributed 10 percent of attacks to the Taliban. At least 40 members of NATO's Afghanistan force have been killed by Afghan soldiers or policemen (these incidents are known as "green-on-blue" attacks) this year, with at least nine slain in the past two weeks.

In one recent attack in Nangarhar province in eastern Afghanistan, an Afghan policeman who had just returned from vacation grabbed a colleague's weapon and opened fire, wounding several members of the NATO-led International Security Assistance Force.

The Taliban boasted that it had carried out the attack. "An infiltrator mujahid turned his gun on American terrorists in the district headquarters," said a statement on the Taliban's Voice of Jihad website. "The brave hero left the headquarters after the successful attack and joined up with Mujahideen of Islamic Emirate in the district."

Taliban leader Mullah Omar recently issued a statement observing that the group had established the "Call and Guidance, Luring and Integration" department "with branches…now operational all over the country" to encourage defections.

The Long War Journal reports that the number of green-on-blue attacks rose from two in 2008 to six in 2010 and 26 this year.

Although NATO commanders have attributed 90 percent of the attacks to personal enmity and cultural differences, the major spike began in 2011, "just after President Barack Obama announced the plan to pull the surge forces, end combat operations in 2014, and shift security to Afghan forces," LWJ said in an analysis published Thursday. "While cultural and personal differences may play a role in the increase in attacks, Taliban and defections by Afghan security personnel who have decided to ingratiate themselves with the Taliban by attacking NATO forces likely play a far more significant role in the green-on-blue attacks than NATO admits."

The Afghan government disagrees with NATO's analysis, blaming foreign spies for the upsurge in attacks on Western forces. But without a far more comprehensive study of the problem, it is impossible to say why the number of attacks is increasing.

NATO officials have responded by setting up a "guardian angel" system of soldiers whose responsibility is to protect their colleagues from attack by Afghan forces who are supposed to be their allies.

Read the full Long War Journal analysis here.