American and Afghan officials are expanding the range of explanations for a surge in "insider attacks" on U.S. troops, adding on Wednesday the theory that the burden of fasting during the Muslim holy month of Ramadan combined with the summer heat may have prompted more Afghan soldiers and police to turn their guns on their American partners.

Whatever the underlying reasons, the attacks are taking a toll and raising questions about the risk of American and other coalition troops working side by side with Afghan troops as advisers, mentors and trainers. The close contact is an essential element of the U.S. strategy for putting the Afghans in the lead combat role as the U.S. prepares to pull out its last combat troops at the end of 2014.

The top commander of coalition forces in Afghanistan, Marine Gen. John R. Allen, said Thursday that while the reasons for the killings are not fully understood, the effect of Ramadan fasting is likely among the causes.

"The idea that they will fast during the day places great strain on them," Allen said, adding that the stress may have been compounded by Ramadan falling during the heat of summer and the height of the fighting season. He acknowledged that hunger and heat are not the primary causes for the killings, but it is among many "different and complex reasons for why we think this may have increased" lately.

He also cited Taliban infiltration of Afghan security forces and personal Afghan grievances against U.S. troops, who Afghans have in some cases accused of being brutish and insensitive to local culture and customs.

Insider attacks have been a problem for the U.S.-led military coalition for years, but it has exploded recently into a crisis. There have been at least 32 attacks so far this year, killing 40 coalition members, mostly Americans. Last year there were 21 attacks, killing 35; and in 2010 there were 11 attacks with 20 deaths.

August has been especially worrisome, with at least 10 insider attacks by Afghans, killing 10 Americans. And they have happened across the country; in the far western province of Farah on Aug. 17, killing two members of a Marine special operations unit; in the southwestern province of Helmand on Aug. 10, killing a total of six Marines in two separate shootings, and in the eastern province of Paktika on Aug. 7, killing one American. The latest was Sunday in Spin Boldak, in southern Kandahar province, where an Afghan police officer opened fire inside a police station, killing a U.S. soldier who was assigned as a police adviser.

The Afghan government asserted on Wednesday that the attacks are the result of Afghan soldiers and police being brainwashed by agents of foreign intelligence services. Allen, speaking from Afghanistan to reporters in Washington, said he had yet to see evidence of that.

"I'm looking forward to Afghanistan providing us with the intelligence that permits them to come to that conclusion so that we can understand how they've drawn that conclusion," Allen said.

Ramadan, during which observant Muslims forgo food and drink in daylight hours, is based on a lunar calendar and covers a slightly different period each year. Prior to its start this summer, U.S. officials made no mention, at least publicly, of its potential to raise the security risk for U.S. and other coalition troops working alongside their Afghan counterparts.

Allen said U.S. officials took precautions during Ramadan and will review what adjustments should be made in the future.

"We were very careful, actually, during Ramadan this year to undertake operations during those times that would not place great physical strain on the troops _ their troops, as well as ours_ given the partnership requirements," he said.

Mark Jacobsen, a defense specialist at the German Marshall Fund of the U.S. and a former senior NATO civilian representative in Afghanistan, said Allen's theory about the role of Ramadan in the insider attacks is "very reasonable."

"However, what we need to understand is that no matter what the source or motivation of the attacks, the insurgents are going to exploit them," he said.

The deadly trend creates a dilemma for U.S. troops who must simultaneously be on guard for insider threats while also trying to train and advise in settings that are inherently dangerous.

John Agoglia, a retired Army colonel who headed a counterinsurgency training center in Kabul in 2008-10, said he recalled that Afghans during Ramadan could be "testy, more argumentative" while fasting.

Agoglia, now an executive at IDS International, which provides cultural awareness training for U.S. troops in Afghanistan, said in an interview that he thinks the insider attacks are partly linked to a Taliban effort to "psychologically dislocate" Afghans from their American trainers and advisers, with limited success so far.

Allen acknowledged that U.S. and Afghan officials have struggled to determine what's behind the rise in attacks. For months U.S. officials said the Taliban were to blame in some cases, but that most of the shootings were triggered by personal grievances against American and coalition troops by Afghan soldiers and police.

In many cases _ possibly 40 percent _ no reason can be determined, according to a U.S. defense official who discussed the matter on condition of anonymity because he was not authorized to speak publicly.

The uncertainty reflects, in part, the fact that most of the shooters are either killed on the spot by return fire or they escape and remain at large. And even when they are captured, U.S. officials say, some shooters offer no coherent explanation. That was the case when an Afghan soldier shot Marine Lance Cpl. Edward J. Dycus of Mississippi, on Jan. 31 at their joint base in Helmand province. No clear motive was ever established, officials said.

Allen said U.S. officials believe that about 25 percent of the shootings can be linked to the Taliban, either through impersonation of an Afghan security force member or through coercing or radicalizing him. Pentagon officials said Allen was referring to 25 percent of cases dating back to 2007. They said only about 10 percent of attacks in 2012 can be blamed on Taliban infiltration.

Asked how these attacks are affecting morale among American troops, Allen said that each killing creates a "moment of crisis." But he added that in most cases the crisis is overcome by focusing on the mission.

"It is not about vengeance or retribution" for the attacks, he said. "It's about gripping the mission. It's about understanding that while an Afghan pulled the trigger, the vast majority of the Afghans they know every day, in fact, are their brothers in this campaign and their brothers in this mission."

In a presentation to Pentagon reporters, Allen offered a cautiously optimistic assessment of the overall war effort.

He said deadly attacks by the Taliban insurgency are affecting an ever-shrinking proportion of the Afghan population.

"The insurgency we face today, while still active, dangerous and capable of inflicting harm, is trying hard to project its strength as its position continues to slowly erode," he said, adding, "We've taken scores of their leaders and fighters off the battlefield, and we've systematically separated the insurgents from more and more of the Afghan population."

Allen is overseeing a steep decline in U.S. troop levels, scheduled to reach 68,000 by the end of September. As of Thursday there were still 81,500 in the country, according to the Pentagon. Allen has said he will wait until late in the year before recommending when additional reductions should be made.

The troop number peaked at 101,000 last year.

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Robert Burns can be followed on Twitter at http://www.twitter.com/robertburnsAP