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.

Karzai's collaboration was needed to "further strengthen ISAF-Afghan cooperation to counter the insider attack threat, including augmented counterintelligence measures, even more rigorous vetting of Afghan recruits, and stepped up engagement with village elders, who often play a key role by vouching for Afghan security personnel," the statement said.

Concern over insider attacks mounted Friday, when two Afghan security personnel opened fire on Western colleagues, killing two U.S. soldiers and causing a number of other casualties in two separate attacks.

The attacks further eroded trust between foreign troops and the Afghans they work with, a week after six American troops were killed in a single day by their local colleagues.

The two Americans were killed in western Farah province, NATO's U.S.-led International Security Assistance Force said. ISAF also confirmed that an unspecified number of foreign and Afghan soldiers were shot and wounded by an Afghan soldier in the southern province of Kandahar.

Panetta said he was "very concerned" about the insider attacks and the impact they are having on cooperation with Afghan allies.

The attacks will likely add to pressure in NATO nations for an exit as soon as possible from the increasingly unpopular war now nearly 11 years old.

[image via Agence France-Presse]