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FILE - In this April 5, 2012, file photo former Rep. Marilyn Musgrave, R-Colo., left, stands with pro-life supporters at a news conference in Concord, N.H. ?There are lots of things that the pro-abortion community will throw at Paul Ryan because there's lots of ammunition since he has such a solid pro-life record,? says Musgrave. (AP Photo/Jim Cole, File)

WASHINGTON — President Barack Obama declared Monday he is sticking to his war strategy of using U.S. troops to advise and mentor Afghan forces, even as a suddenly growing number of Americans are being gunned down by the very Afghans they are training to take on insurgents.

In just the past 10 days, Afghan forces have attacked their coalition partners seven times, killing nine Americans. For the year there have been 32 such incidents, killing 40, compared to 21 attacks killing 35 troops in all of 2011.

"We are deeply concerned about this, from top to bottom," Obama said. But he said the best approach, with the fewest number of deaths in the long run, would be to stick to the plan for shifting security responsibilities to the Afghans.

"We are transitioning to Afghan security, and for us to train them effectively we are in much closer contact — our troops are in much closer contact with Afghan troops on an ongoing basis," Obama said. "Part of what we've got to do is to make sure that this model works but it doesn't make our guys more vulnerable."

That vulnerability, however, has been exposed in a strikingly deadly way in recent days.

U.S. officials offer two main theories for why Afghan security forces are turning their weapons on Western partners: infiltration by the Taliban and a U.S.-Afghan culture clash.

Both of those root causes suggest that the problem may get worse as American and other coalition forces shift further into an adviser/mentor role. And that, in turn, raises questions about U.S. ability to train and shape the Afghans into a force that can stand up to the Taliban insurgency in 2014.

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Jacqueline L. Hazelton, a visiting assistant professor at the University of Rochester, who has extensively studied counterinsurgency strategy, sees the attacks stemming from a combination of Afghan resistance and resentment.

"As disturbing as the attacks are as a Taliban tactic, the broader popular anger revealed — among those the mission is supposed to be most closely allied with and most directly useful to — is even more dangerous for the longer term and reveals a greater rot within," Hazelton said in an email exchange.