NATO's US-led mission in Afghanistan said on Monday there had been failures in Afghan security procedures meant to weed out potential killers of Western troops before they join Kabul's forces.

Since January 1, seventeen foreign troops, including seven Americans and five French trainers, have been shot dead by Afghan personnel in 10 separate attacks. The fatalities represent more than one in six of ISAF's 96 fatalities.

The International Security Assistance Force (ISAF), which is training Afghans to take over responsibility for security for the whole country by the end of 2014, said the deaths had sapped spirits among its troops.

"Although the incidents are small in number we are aware of the gravity they have as an effect on morale," ISAF spokesman Brigadier General Carsten Jacobson said in Kabul.

"Every single incident has an out-of-proportion effect on morale and that goes for coalition forces as it goes for Afghan national security forces."

Recruits to the Afghan forces undergo an eight-step vetting process carried out by Afghan authorities, including identification verification, recommendations and criminal background checks.

But Jacobson said investigations into the shootings had found lapses.

"What we have found in individual cases is that there was a mistake done here, or there, or there," he told reporters.

"The identity papers weren't checked properly, the papers that were coming from village elders were not sufficient, drug tests were not taken regularly or sufficiently or something like that.

"Wherever we see that we take that as measures to be taken and improved."

Asked if some of the incidents could have been prevented, he responded: "Afterwards you always know that you shouldn't have had that car accident."

ISAF has around 130,000 soldiers fighting alongside 350,000 Afghan security personnel in a bid to help President Hamid Karzai's government reverse a Taliban-led insurgency.

Among the measures being taken, Afghanistan's intelligence services are hiding agents among new recruits at the country's army and police training schools to try to spot potential gunmen, NATO said.

ISAF has also taken several security measures in response to the shootings, including assigning "guardian angels", soldiers who watch over their comrades as they sleep.

The concept "makes sure that soldiers are not without protection at any stage", said Jacobson.

He dismissed repeated claims by the Taliban that they were behind the attacks.

"The insurgency is claiming nearly every single incident for itself. Our findings are that in the vast majority... personal grievances are one of the major causes."

Insurgent involvement had been proved in only a few cases, he said.

Instead, there were a "number of other causes including stress syndromes on soldiers who are living in a country that has been 30 years at war".

An Afghan policeman poisoned and shot dead nine of his colleagues in the eastern province of Paktika last week.

As a matter of leadership Afghan commanders need to keep a close eye on their soldiers to try to spot potential problems before they happen, Jacobson added.

"A soldier that has seen considerable battle stress has to be observed and a soldier who hasn't been on leave for a long time has to be looked at whether it is time to give them a break," he said.

"Somebody who has been on leave with his family in a refugee camp in Pakistan has to be looked at when he comes back, soldiers who have problems at home, financial problems."

A spokesman for the Afghan defence ministry declined to comment.

The relationship between Afghan and NATO forces is vital but tensions have also mounted this year due to a series of incidents at the hands of US troops.

A video emerged of US forces urinating on Taliban corpses, copies of the Koran were burnt on a major US military base and an American sergeant has been charged with 17 counts of murder over a massacre of civilians.

Jacobson said he was not drawing parallels between the so-called green-on-blue killings and the case of Staff Sergeant Robert Bales, the US soldier who has been charged over the March 11 massacre.

Bales has been reported in the United States to have been suffering from financial problems.