An Afghan soldier opened fire on NATO troops in eastern Afghanistan on Friday, killing an American and leaving two others wounded, officials said.
The death brings to 20 the number of NATO soldiers killed by Afghan colleagues in at least 15 separate attacks so far this year.
The shootings have forced US-led troops to step up security and fanned tensions between Western soldiers and the Afghan security forces they are supposed to be training to take responsibility for the country by 2014.
"An Afghan army soldier turned his weapon against US soldiers inside an Afghan-US military base in Kunar province, killing one US soldier and injuring two others," said provincial police chief Ewaz Mohammad Naziri.
The attack took place early Friday and the Afghan managed to flee the scene, Naziri said.
NATO's mission in Afghanistan confirmed that a man dressed in local army uniform "turned his weapon against coalition service members" and killed one soldier.
But the International Security Assistance Force gave no further details and said the nationality of the dead soldier would be announced by his home country. Nor does it list its wounded in any attack.
The incident was under investigation, it said.
An increasing number of Afghan soldiers are turning their weapons against NATO troops who are helping Kabul fight a decade-long insurgency by hardline Taliban Islamists.
Some of the assaults 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.
ISAF has taken several security measures in response to the shootings, including assigning "guardian angels" -- soldiers who watch over their comrades as they sleep.
Aside from the "green-on-blue" shootings, relations between the United States and its Afghan allies have been rocked this year by a series of incidents involving American 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.