An Afghan special forces soldier killed an American when he opened fire on US troops in southern Afghanistan, military spokesmen said Thursday, the latest in a series of such attacks.

The soldier's death takes the toll of foreign troops killed by Afghans they were working with to 18 this year -- including 10 Americans and five French trainers -- in 11 separate attacks.

"An officer of the Afghan special forces opened fire on a group of American soldiers in Shah Wali Kot district of Kandahar province, killing one US soldier," regional Afghan corps commander Abdul Hamid Hamid told AFP.

"The officer was himself killed when the Americans returned fire," he said.

A spokesman for US forces in Afghanistan and a defence official in Washington confirmed the incident, without giving details.

The shooting is the latest in a series of so-called green-on-blue attacks in which Afghan soldiers have turned their weapons against NATO troops helping Kabul fight a decade-long insurgency by hardline Taliban Islamists.

NATO's International Security Assistance Force (ISAF), which is training Afghans to take responsibility for security for the whole country by the end of 2014 when most foreign troops pull out, has said the deaths sap spirits among its soldiers.

"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 earlier this month.

"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."

Some of the attacks are claimed by the Taliban, who say they have infiltrated Afghan army ranks, but many are attributed to cultural differences and antagonism between the allied 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."

Among new measures taken to avoid similar incidents, Afghanistan's intelligence services are hiding agents among recruits at the country's army and police training schools to try to spot potential killers, 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.

Relations between the US and its Afghan allies have also been rocked this year by incidents involving 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.

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

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