US Defense Secretary Leon Panetta has made a phone call with Afghan President Hamid Karzai to discuss recent spate of "insider" attacks that has plagued the security transition in the country, the Pentagon said Sunday. According to a statement from Pentagon Press Secretary George Little, Panetta and Karzai discussed on Saturday "the challenge of insider attacks against Coalition and Afghan forces." They expressed shared concern over the issue and "agreed that American and Afghan officials should work even more closely together to minimize the potential for insider attacks in the future," the statement said. Panetta advised Karzai to continue working closely with NATO troops Commander John Allen, as well as augmenting counterintelligence measures, and more rigorous vetting of Afghan recruits. The U.S. top defense official also suggested that measures be taken to step up engagement with village elders, who often play a key role by vouching for Afghan security personnel. "Insider" attacks, sometimes referred to as "green on blue" attacks, have been on the rise recently in Afghanistan. According to NATO figures, attackers in Afghan security force uniforms have killed 39 international troops in 29 such attacks so far this year.