Afghanistan's Defense Ministry on Wednesday said it has detained or removed hundreds of soldiers from its military's ranks as part of an investigation into the backgrounds of its troops after a surge in insider attacks against foreign forces.

Defense Ministry spokesman Mohammad Zahir Azimi did not provide any other details or an exact numbers. He said many of the soldiers affected had been pushed out because they had either incomplete or forged documents. He did not say whether any were connected to the Taliban or other insurgent groups.

Azimi added that the investigation began about six months ago but could provide no further details. He said more information would be released next week.

NATO did not comment on Azimi's remarks. But on Sunday, the deputy commander of the U.S.-led NATO coalition, British Lt. Gen. Adrian Bradshaw, also confirmed that a number of soldiers were suspended. He did not provide a number.

The United States and its allies are pushing to have Afghan forces take over security for the country by the end of 2014. That effort has been imperiled by the spike in insider attacks, which have killed 45 international service members this year, most of them Americans. There were at least 12 such attacks in August alone, resulting in 15 deaths.

The killings are straining an alliance already stretched by a tense relationship with Afghanistan's mercurial president, Hamid Karzai, and disagreements over NATO tactics that Kabul says endanger civilians.

NATO Secretary-General Anders Fogh Rasmussen expressed deep concern to Karzai on Wednesday over the growing number of assaults by Afghan troops on coalition forces.

Spokeswoman Carmen Romero said Karzai had assured Fogh Rasmussen in a phone conversation that he was doing all he could to stop the attacks.

U.S. Gen. John R. Allen, who commands NATO's 129,000-strong force, briefed the alliance's top decision-making body on Wednesday about the shootings.

Coalition authorities have said about 25 percent of this year's insider attacks had confirmed or suspected links to the Taliban. The militants have sometimes infiltrated the ranks of the Afghan army and police and in other cases are believed to have coerced or otherwise persuaded legitimate members to turn on their coalition partners.

The insider killings, the relentless violence around Afghanistan and the number of foreign troops dying _ 320 so far this year _ seem to run against NATO's narrative that its strategy in the country has so far been a success and that its plan to hand over security to local forces is on track.

In more violence Wednesday, NATO said two of its service members died in a helicopter crash in eastern Afghanistan. It did not provide any other details or the nationalities of the two, but said the crash was under investigation. Their deaths brought to six the number of foreign troops who have been died this month.

The Taliban claimed to have shot down a helicopter in eastern Logar province, where coalition forces are staging an operation against the insurgents. The Taliban often issue claims of responsibility even when it is unclear whether they had anything to do with the downing of an aircraft.

Bradshaw, the deputy commander of the U.S.-led NATO coalition, also said in a statement Sunday that the U.S. military had halted the training of some Afghan forces. Although the move affects only about 1,000 trainees in the Afghan Local Police, a small fraction of the country's 350,000-strong army and police, it highlighted the potential of the attacks to derail the U.S.-Afghan handover of security considered so essential to the international exit strategy.

He added that the decision was "part of a number of actions recently instituted to reinforce existing precautions related to the insider threat."

He said much of the re-vetting task had already been finished and that some individuals had been suspended pending further investigation, or removed from the force.

Bradshaw made the remarks after meeting with Karzai to discuss ways to counter the threat posed by insider attacks.

On Tuesday, Fogh Rasmussen said the insider shootings will not derail plans to draw down international troops from Afghanistan, but in the meantime, NATO will "do everything it takes" to stop such killings.

Additional measures to prevent such attacks may include strengthened vetting and screening procedures, improved counterintelligence, as well as cultural awareness training, Fogh Rasmussen said. He did not elaborate.

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Associated Press writers Patrick Quinn in Kabul and Slobodan Lekic in Brussels contributed to this report.