KABUL, Afghanistan (AP) -- NATO said Monday that it has scaled back operations with Afghan soldiers and policemen to lower the risk of insider attacks and reduce local tensions over an anti-Islam video that prompted protests in Afghanistan.

It's the second order that curbs contact between foreign troops and their Afghan partners, undermining the mantra that both sides are fighting the Taliban "shoulder to shoulder." The directive could jeopardize the U.S.-led coalition's key goal to get Afghan forces ready to take over security from foreign forces by the end of 2014 -- just 27 months from now.

Until now, coalition troops routinely conducted operations such as patrolling or manning outposts with their Afghan counterparts. Under the new rules issued on Sunday by Lt. Gen. James Terry, such operations are no longer routine and require the approval of the regional commander.

Insider attacks have spiked in recent months.

So far this year, 51 international troops have been killed by Afghan forces or militants wearing their uniforms -- a development that has fractured the trust between NATO troops and their Afghan allies. The disturbing trend comes as Afghans chanting "Death to America" have staged several recent protests against an anti-Islam film produced in the U.S. The film also sparked demonstrations in other nations.

A protest in Kabul over the film that mocks the Prophet Muhammad turned violent Monday, with hundreds of men torching tires, cars and shipping containers and lobbing rocks at a U.S. base on the edge of the capital. More than 20 police officers were injured by rocks before the protesters were finally dispersed by officers shooting in the air, officials said.

"Recent events outside of and inside Afghanistan related to the 'Innocence of Muslims' video, plus the conduct of recent insider attacks, have given cause for ISAF troops to exercise increased vigilance and carefully review all activities and interactions with the local population," said coalition spokesman Jamie Graybeal.

Earlier this month, the U.S. military stopped training about 1,000 members of the Afghan Local Police, a controversial network of village-defense units that is growing but remains a fraction of the country's army and police force, which will soon be 352,000 strong.

The coalition downplayed the impact of the directive, saying international forces had not stopped partnering and advising Afghan forces. Coalition officials said the directive was given at the recommendation of -- and in conjunction with --key Afghan leaders.

U.S.-led coalition companies remain partnered with Afghan units, but have changed the way they conduct their daily partnering operations, the coalition said.

"In the past, elements of a company routinely conducted operations -- like patrolling or manning an outpost -- with elements of the Afghan battalion," the coalition said in emailed statements to The Associated Press.

Under the directive, these operations are no longer routine and now require the approval of the general in charge of their regions.

The order will be in place for an undetermined period of time, according to Lt. Col. Rich Spiegel, chief public affairs officer for the coalition's operational command.

"It doesn't mean we're walking away from these units. We can advise from the next level up," he said. "It means we may not be out on patrol with them."

Both the U.S. and Afghan authorities have already taken other steps to lessen the risks of attacks.

Earlier this year, the U.S. commanders assigned some troops to be "guardian angels" who watch over their comrades in interactions with Afghan forces and even as they sleep. The U.S. also started allowing Americans to carry weapons in several Afghan ministries and made security more of a consideration in evaluating visits to Afghan government offices. U.S. officials also recently ordered American troops to carry loaded weapons at all times in Afghanistan, even when they are on their bases.

Afghan authorities recently detained or removed hundreds of soldiers because they had submitted incomplete or forged documents to the military. The Defense Ministry did not say whether any were connected to the Taliban or other insurgent groups, but noted that some were suspected of having had contacts with militants.

On Sunday evening, the coalition said an Afghan soldier fired on a vehicle he believed was driven by NATO soldiers on a shared base in southern Afghanistan, slightly wounding a foreign civilian worker. The soldier turned his weapon on a vehicle that was driving inside Camp Garmser, a shared base in Helmand, said NATO forces spokesman Maj. Adam Wojack. Another Afghan soldier disarmed the attacker and took him into custody. The assailant told interrogators he had thought he was targeting troops, Wojack said.

That shooting came the same day an Afghan police officer shot and killed four American service members in Zabul, also in the south. That followed a shooting Saturday in which a man wearing the uniform of a government-backed militia group killed two British soldiers in Helmand province.

The insider assaults drew unusually strong comment Sunday from the U.S. military's top officer, Army Gen. Martin Dempsey, who called the problem of rogue Afghan soldiers and police attacking allied troops "a very serious threat" to the war effort. Dempsey said something has to change in order to address the escalating problem, suggesting that Afghans need to take the matter as seriously as the Americans do.

But U.S. and NATO officials appeared to be stepping back from that assessment on Monday. U.S. Defense Secretary Leon Panetta said in a visit to Tokyo that the insider attacks are a "last gasp" of a Taliban insurgency that has not been able to regain lost ground. And the top spokesman for the coalition in Afghanistan, German Brig. Gen. Gunter Katz, told reporters in Kabul that Afghan officials were taking appropriate measures to try to prevent attacks.

"The Afghans started, according to their own reports, a re-vetting of their own soldiers and policemen and have already relieved a couple of hundred from active duty," Katz said. He also noted that the Afghan forces have increased training about cultural differences between the two forces.

Katz said the quick reaction of Afghan forces to the attack at Camp Garmser showed that they were taking such attacks seriously. "It was members of the Afghan national army who reacted instantly and detained that shooter," Katz said.

A spokesman for Afghan President Hamid Karzai said the Afghan government is making every effort to stop the attacks, noting that Karzai has made it a priority in meetings.

"The president himself has taken this issue very seriously," spokesman Aimal Faizi said.

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Associated Press writers Heidi Vogt, Amir Shah and Patrick Quinn in Kabul and Mirwais Khan in Kandahar contributed to this report.