The last insider attack occurred in August of 2017.
The attack took place in southern Afghanistan on Saturday, a statement from the Nato-led Resolute Support mission said.
It was not immediately clear what became of the shooter. The Afghan province that has seen the highest number of attacks is Helmand to the south, a Taliban stronghold and the heart of Afghanistan's opium-growing industry. Western officials say most insider attacks stem from personal grudges and cultural misunderstandings rather than insurgent plots.
The name of the killed soldier has not been released.
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"Cpl. Maciel was an excellent soldier beloved by his teammates and dedicated to our mission", Maciel's battalion commander, Lt. Col. David Conner, said in the 3rd Infantry Division statement.
He said the strategy announced previous year by President Donald Trump of sending more troops to increase pressure on the Taliban and push them toward negotiations was working, and would reassure Afghans "that we will support them as they continue fighting to liberate their country and their people".
The Taliban have been battling USA -led North Atlantic Treaty Organisation forces in an attempt to restore their version of strict Islamic law in the country after being toppled by a US -led campaign in 2001.
United States should continue military exercises with South Korea if North Korea talks break down GOP senator: Actions toward North Korea will speak louder than words Graham: "China's hands" all over North Korea rebuke of USA negotiators MORE on Monday said the U.S. would be willing to take part in peace talks between Afghanistan and the Taliban. Efforts to impose a ceasefire in the region have been mostly unsuccessful.
Pompeo said the Afghan people deserve peace and the government should do more to end the 17-year war despite the Taliban repeatedly rejecting calls for peace talks.