WASHINGTON: US Defense Secretary Leon Panetta said on Tuesday he was “very concerned” about attacks on US and Nato troops by their Afghan allies and the impact they are having on cooperation.

“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,” he said.

“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 told reporters.

The attacks have raised questions about the viability of US and Nato efforts to train Afghan security forces so they will be ready to take charge of security after the departure of foreign forces in 2014.

In the latest 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.

Seven soldiers of the US-led International Security Assistance Force (ISAF) lost their lives over four days last week when Afghan forces turned weapons on their Nato allies.

According to Nato, 37 of its soldiers have died in 27 such attacks so far this year, up from 21 incidents and 35 deaths in all of 2011.