AUCKLAND, New Zealand -- The 33,000 additional U.S. troops that President Barack Obama ordered to Afghanistan to tamp down the Taliban attacks nearly two years ago have now left the country, but a new wave of deadly insider attacks and a reassessment of how NATO troops partner with Afghans have raised questions about how well the military strategy is working.

U.S. Defense Secretary Leon Panetta announced on Friday the troops had come out, declaring the surge had accomplished its mission.

But after a tumultuous week in Afghanistan that saw commanders put limits on when NATO and Afghan troops can patrol together, Panetta also acknowledged there will still be difficult days ahead.

"The surge did accomplish it objectives of reversing the Taliban momentum on the battlefield and dramatically increasing the size and capability of the Afghan national security forces," Panetta told reporters at a press conference at the Government House here where he was meeting with New Zealand leaders.

He said the re-deployment of the 33,000 troops was a "very important milestone" and that the U.S. is on track to accomplish its goals in Afghanistan. The withdrawal, which was completed on schedule, still leaves close to 100,000 NATO troops there, including 68,000 Americans.

Panetta's success mantra, however, is called into question by the decision earlier this week that, at least temporarily, NATO operations with small-sized Afghan units are no longer routine, and will require the approval of the regional commander.

Until now, coalition troops routinely conducted operations such as patrolling or manning outposts with small units of their Afghan counterparts. But a growing wave of in so-called insider attacks in which Afghan Army and police troops, or insurgents dressed in their uniforms, have been turning their guns on U.S. and NATO forces, has shaken the trust between the allied troops and the Afghans they are there to train.

And it's called into question the core strategy that relies on NATO troops working shoulder to shoulder with Afghans, training them to take over the security of their own country so the U.S. and its allies can leave at the end of 2014 as planned.

As of this week 51 coalition troops have been killed in such attacks this year.

Australian Brig. Gen. Roger Noble, deputy to the alliance's operations chief, acknowledged earlier this week that the attacks are rattling the troops.

"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."

Panetta, however, has rejected suggestions that the strategy is failing, and on Friday he said "we have turned the corner," in Afghanistan and have successfully been able to build up the Afghan security forces so they can take the lead in security for large sections of the country.

Panetta said the top U.S. commander in Afghanistan, Gen. John Allen, "is saying that the force he has put in place is sufficient to accomplish that mission."

The number of U.S. forces there peaked at about 101,000 last year, and they have been coming out slowly over the past several months.

The surge was aimed at beating back the Taliban to give the Afghan government and its security forces the time and space to take hold. The key goal was to ensure the Taliban did not regain a foothold in the country that could allow it once again to become a safe haven for terror groups. And there was hope that Taliban leaders would be willing to come to the peace table.

Military commanders say they have made broad gains against the Taliban, wresting control of areas where the insurgents once had strong footholds. And Panetta has characterized the insider attacks as the last gasp of a desperate insurgency.

But other top military leaders, including U.S. Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, are worried about the impact of the attacks on the troops. Dempsey called them a "very serious threat" to the war campaign and has declared that "something has to change."