WASHINGTON -- The government of Afghanistan has begun to take a series of enhanced measures to prevent the killing of NATO soldiers by Afghan forces, Minister of Defense Abdul Rahim Wardak said on Tuesday.

Speaking to reporters at the embassy of Afghanistan, Wardak ran off a long list of steps taken by his country's armed forces to halt a slew of recent incidents in which Afghan security forces, trained and equipped by NATO forces, have turned their guns on the trainers. The incidents are known as "green on blue" attacks by the U.S. military.

"We are taking a lot of preventative measures," Wardak said, adding that the attacks were "very tragic." "We are doing much better vetting in our recruiting centers, requiring two individuals as guarantors for every applicant and are doing full background checks on all of our soldiers."

Afghanistan's military is raising the security level within the units, restricting mobile phone usage for soldiers and instituting new drug test requirements, Wardak also said. Some 150,000 Afghan soldiers have already undergone enhanced background checks, Wardak said.

"That is just a short number of the measures we are taking," he said.

Green-on-blue killings have risen to the forefront of American and Afghan officials' concerns after a spate of incidents this year suggested that they may be on the rise or part of a concerted effort by the Taliban to disrupt America's withdrawal plan. The Americans' exit ultimately hinges on preparing Afghan forces to take over national security.

In February, two American officers attached to the Afghan Interior Ministry were shot point-blank while at work inside the ministry compound, prompting NATO nations to pull out their forces from all Afghan government facilities.

A new visual project by the New America Foundation tracking incidents of green-on-blue violence has identified 34 separate incidents in which 81 NATO troops were killed by the Afghan army or police.

Fifty-two of those killed in this fashion since 2007 have been Americans, according to U.S. officials; this tally includes 10 incidents this year. The vast majority of green-on-blue attacks have occurred in the past two years.

"This pattern of attacks raises a fundamental problem for the plans of the United States and other NATO countries to draw down their forces over the next two years," wrote New America's Peter Bergen in an introduction to his group's visual project.

In a speech late last month at the Brookings Institution, Marine Corps General John Allen, the top U.S. commander in Afghanistan, acknowledged that the attacks had caused "an erosion of trust" between American and Afghan forces.

Allen later suggested to reporters that such killings were largely the result of infiltration of Afghan ranks by anti-American elements and that a continuation of such incidents was inevitable.

“The enemy is going to do all that they can to disrupt our operations and to disrupt the integrity of the government forces,” Allen said. “We should expect this.”