Pentagon chief Leon Panetta urged Afghan President Hamid Karzai on Saturday to crack down on a rise in attacks by Afghan security personnel on their Western colleagues, officials said.

U.S. concern is mounting over the unprecedented number of such “green-on-blue” attacks, which have now killed 39 international troops in 29 such incidents so far this year, according to NATO figures.

During a telephone call, Panetta and Karzai “expressed shared concern” over the challenge of insider attacks against coalition and Afghan forces, Pentagon spokesman George Little said in a statement.

The pair “agreed that American and Afghan officials should work even more closely together to minimize the potential for insider attacks in the future,” he added.