One American service member was killed and two others were wounded on Saturday in what the USA -led coalition described as an "apparent insider attack".
Around 16,000 troops from U.S., North Atlantic Treaty Organisation and other partner countries are in Afghanistan, training the Afghan forces within the framework of a NATO-led mission.
The unidentified American was killed on Saturday in the southern part of the country, according to a statement from NATO's Resolute Support mission there. "The incident is under investigation".
It was not immediately clear who carried out the attack, or whether they had been apprehended.
The incident happened at an Afghan army base in Uruzgan Province in southern Afghanistan, according to a U.S. official. But as NPR's David Welna reported last week, a "public information blackout" by the Trump administration has meant that, "since late previous year, the Pentagon's stopped posting those numbers for Iraq, Syria and Afghanistan".
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The Taliban have also released a statement, saying an Afghan soldier opened fire on U.S. soldiers in Uruzgan.
The incident comes just over a year after three USA soldiers were killed and one other was injured when an Afghan National Army soldier fired on a unit in Nangarhar Province.
North Atlantic Treaty Organisation said an investigation of the attack was underway.
The last insider attack occurred in August 2017. A U.S. Defense Department report released earlier this month found civilian casualties had increased 73 percent between December and May compared to the same period the year before.
The force of 16,000 troops includes service members from 39 North Atlantic Treaty Organisation nations and allies.