US-led NATO forces in Afghanistan said Monday that attacks by Afghan security personnel on their Western allies had had a disproportionately large effect on troop morale.

So far this year 17 members of the US-led International Security Assistance Force (ISAF) have been killed in 10 so-called "green on blue" attacks by their Afghan counterparts in the fight against the Taliban-led insurgency.

ISAF has taken a number of security measures in response, including the concept of the "guardian angel", a soldier assigned to watch over his comrades to protect them as they sleep.

The concept "makes sure that (soldiers) are not without protection at any stage", said ISAF spokesman Brigadier General Carsten Jacobson.

"Although the incidents are small in number we are aware of the gravity they have as an effect on morale," he told a press conference in Kabul.

Jacobson dismissed repeated claims by the Taliban, the main militant group waging an insurgency against President Hamid Karzai's Western-backed government, that they were behind the attacks.

"We do not believe that this string of recent events is linked or part of any larger coordinated effort of the insurgency," he said.

"We believe that each is an individual case and each one has its own underlying circumstances and motives as to the reason why they occurred."

The causes could include stress and fatigue, family issues or financial problems, and commanders needed to keep a close eye on their soldiers to try to spot potential problems before they happened, he added.