WASHINGTON -- A series of "insider attacks" against U.S. and allied troops by Afghan forces are an attempt by the Taliban to drive a wedge between coalition and Afghan troops, a senior officer said Wednesday. But he said that while Western troops are now warier of Afghan partners, they are determined to avoid a full breakdown in trust.

Australian Brig. Gen. Roger Noble, deputy to the alliance's operations chief, acknowledged in a teleconference from Kabul that the attacks, which have killed 51 coalition troops this year, are upsetting the troops. He said he visited Australian troops at a base where an insider attack killed three Australians on Aug. 29.

"They were pretty unhappy," Noble told reporters at the Pentagon, adding that they also recognize the potential for the attacks to damage the war effort.

"They live and breathe with Afghans, and they know that most of the Afghans they're with think that sort of conduct is abhorrent, as well," he added. "So the enemy's got a tough task in actually driving a wedge between us, because we know what the people that do this are trying to do, and we will stand firmly against it."

Noble said it's understandable that these attacks are particularly galling to the families of those killed and wounded.

"It strikes right at the heart of our resolve," he said. "It's one thing to be killed in action by the insurgents. It's quite another to be shot in the back of the head at night by your friends."

Reflecting frustration among some in Washington over recent setbacks in Afghanistan, Sen. John McCain, R-Ariz., suggested the U.S. consider getting out earlier than the planned 2014 exit.

"I think all options (should) be considered, including whether we have to just withdraw early rather than have a continued bloodletting that won't succeed," McCain told reporters on Capitol Hill. "The whole program has to be re-evaluated because the process they said would lead to that (December 2014) withdrawal has been an abject and total failure."

In response to the spike in insider attacks and tensions over an anti-Muslim movie produced in the U.S., coalition leaders in Kabul ordered that military operations with small-sized Afghan and coalition units be undertaken only with the approval of the regional commander. Previously, coalition troops routinely conducted operations such as patrolling or manning outposts with small units of their Afghan counterparts.

It's unclear whether the coalition's exit strategy can succeed with less partnering with Afghan policemen and soldiers, who are scheduled to take over for foreign combat troops by the end of 2014.

Gen. Martin Dempsey, chairman of the U.S. Joint Chiefs of Staff, said in Washington on Wednesday that the changed approach to working with Afghan forces should be seen as a temporary adjustment, not a change in war objectives or strategy.

"The campaign objectives that we've established are accurate, they are sound, they are prudent and they are feasible, and they are achievable," he said. "But we don't get from here to there in a straight line, we never do."

Asked about the matter while traveling in China on Wednesday, Defense Secretary Leon Panetta echoed Dempsey's remarks.

"Our fundamental strategy remains the same," Panetta said. "These tactics and what's occurring here is aimed at one thing, it's aimed at trying to break the relationship between the United States and the Afghan army, which is critical for our ability to ultimately move toward security in the future in Afghanistan."

There have been at least three insider attacks this month, including one on Monday in which no one was killed.

The Pentagon on Wednesday released the names of four U.S. soldiers killed in an insider attack on Sunday, although its announcement did not mention that they were shot by Afghan troops. It just said "their position was attacked with small-arms fire."

The four are Sgt. Sapuro B. Nena, 25, of Honolulu; Spc. Joshua N. Nelson, 22, Greenville, N.C.; Pfc. Genaro Bedoy, 20, of Amarillo, Texas; and Pfc. Jon R. Townsend, 19, Claremore, Okla.

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Associated Press writers Lolita C. Baldor in Beijing and Sagar Meghani and Donna Cassata in Washington contributed to this report.

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