WASHINGTON (AFP) ― Attacks by Afghan soldiers on NATO troops have resulted in an “erosion of trust” between the two allies, but ties remain strong and officers on both sides are working to contain the problem, the commander of U.S.-led forces said Monday.

Speaking hours after two British troops and a U.S. soldier were killed in two separate “insider” assaults by Afghan security forces, U.S. General John Allen acknowledged the incidents were cause for concern.

“There is an erosion of trust that has emerged from this. But I believe that the relationship is very strong nonetheless,” Allen, the top commander of NATO and U.S. troops in Afghanistan, told an audience at the Brookings Institution, a Washington think tank.

“For every one of these that occurs, the numbers of interactions that our troops have every single day, with the ANA (Afghan National Army) and ANP (Afghan National Police) forces, can be measured in the tens of thousands,” he said.

Afghan officials were bolstering vetting procedures for recruits and the country’s intelligence service was helping commanders look out for potential extremists or Taliban moles infiltrating the army or police, he said.

More than one in six of the 91 foreign soldiers who died in Afghanistan in 2012 have been killed in so-called “green-on-blue” attacks by Afghan security troops. Monday’s attacks brought the toll for insider attacks this year to 16.

A planned withdrawal of U.S. and coalition forces by the end of 2014 hinges on building up Afghan army and police, but the surge in “fratricidal” attacks threatens to undermine that strategy, with strained relations between NATO troops and Afghan forces marked by distrust and cultural clashes.

Allen said the “insider threat” was typical of counter-insurgency warfare, and that U.S. forces faced similar problems in Iraq and Vietnam.

Although the Taliban has taken credit for the “green-on-blue” assaults, Allen said a majority of the incidents have not been the result of infiltration by the insurgency.

“From our investigations they have accounted for less than 50 percent” of the incidents, Allen said. The attackers “are generally self-radicalized,” he said.

Defense Secretary Leon Panetta, speaking to reporters en route to Ottawa, said Monday he was confident that measures taken by Allen would help limit the problem.

“I don’t think they reflect any kind of broad pattern,” Panetta said.

“We have to take every step possible, working with the Afghans, to make sure that these kinds of incidents are controlled.”

Allen acknowledged that the inadvertent burning of Qurans at U.S. base last month, a video of U.S. troops urinating on the corpses of slain insurgents and a shooting rampage by a U.S. soldier all could influence the outlook of a potential Afghan attacker.