A US soldier was killed and two others wounded in an "apparent insider attack" in southern Afghanistan on Saturday, NATO said.
The so-called "green-on-blue" attack is the latest in a line of incidents where Afghan soldiers have turned their weapons on international forces they are working with.
"The wounded service members, who are in stable condition, are currently being treated," NATO's Resolute Support mission in Afghanistan said in a statement.
"The incident is under investigation."
NATO did not release the identity of the American soldier killed or provide further details about the incident.
The shooting happened at the airport in Tarinkot, the capital of Uruzgan province, a local police officer told AFP.
That information could not be immediately verified by Afghan or NATO officials.
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The Taliban said on Twitter that "a patriot Afghan soldier opened fire on Americans in Uruzgan airport killing and wounding at least four American invaders".
Currently, there are about 14,000 US troops in Afghanistan, providing the main component of the NATO mission there to support and train local forces.
Some of the US forces are involved in counterterrorism operations, particularly against the local Islamic State group.
American casualties have fallen dramatically since the withdrawal of US-led NATO combat troops at the end of 2014.
On New Year's Day a US soldier was killed and four others wounded in combat operations in the eastern province of Nangarhar.
Western officials say most insider attacks stem from personal grudges and cultural misunderstandings rather than insurgent plots.
The latest incident comes as LieutenantGeneral Scott Miller is set to become the next commander of US and NATO forces in Afghanistan.
He will take over from General John Nicholson, who is rotating out of his post after a two-year deployment.
Miller's deployment to Afghanistan, which would see him promoted to four-star general, comes at a time when local security forces are still struggling to contain a resurgent Taliban.
IS is also maintaining its eastern and northern footholds despite an intensified aerial bombing campaign by Afghan and US forces.