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.

Panetta also encouraged Karzai to work closely with General John Allen, who heads the international coalition force ISAF in Afghanistan.