US Defense Secretary Leon Panetta said Tuesday he was "very concerned" about the rise in "insider attacks" on US and NATO troops, and the impact they are having on cooperation with Afghan allies.

In the latest "green-on-blue" attack, an Afghan policeman shot and wounded two troops from the NATO-led mission on Monday in the eastern province of Nangarhar, where Americans troops make up the bulk of the force.

The attacks -- which left seven dead last week -- have raised questions about the viability of US and NATO efforts to train Afghan security forces so they will be ready to take charge once foreign forces leave in 2014.

"Our enemies have attempted to undermine the trust between the coalition and the Afghan forces and in particular they have tried to take credit for a number of so-called green-on-blue or insider attacks that are taking place this fighting season," Panetta told a press conference.

"Make no mistake about it, I've been very concerned about these incidents ... because of the lives lost and because of the potential damage to our partnership efforts," he said.

The Taliban are "resorting to these kinds of attacks to create havoc" because they have been unable to regain any lost territory, the defense secretary added.

According to NATO figures, 37 members of the US-led International Security Assistance Force (ISAF) have died in 27 such attacks so far this year.

That toll has already surpassed the 35 deaths recorded in 21 "green-on-blue" incidents in all of 2011.

Panetta said General John Allen, who heads the international coalition force in Afghanistan, was meeting with Afghan security ministers, and village elders, to discuss further steps to protect against such attacks.

These measures include increasing the intelligence presence to get better information about potential attacks and implementing a notification process, Panetta said.

In particular, he referenced a so-called guardian angel program, "which involves identifying one individual who stands to the side so that he can watch people's backs and hopefully identify people that would be involved in those attacks."

Some of the "green-on-blue" attacks are claimed by the Taliban, who say they have infiltrated the ranks of Afghan security forces, but many are attributed to cultural differences and antagonism between local and US-led allied forces.

"What we identified was that most of them were caused by personal grievances and stress situations," the chief spokesman for ISAF, Brigadier General Gunter Katz, told AFP last week.

"Those isolated incidents don't reflect the overall security situation in Afghanistan. As we speak 500,000 soldiers and policemen are working together to contribute to a more secure and stable Afghanistan," he said.

"We are confident that the morale (among international troops) is still good and those incidents will not affect our transition process."

Panetta meanwhile said cross-border cooperation was increasing with Pakistan, which reopened its border to NATO supply trucks headed for Afghanistan earlier this summer.