A U.S. service member was killed and another was injured in an insider attack on Wednesday in Kabul, according to NATO.
A U.S. civilian was also killed and two more American civilians were wounded in the incident in the capital of Afghanistan, according to a release Wednesday from NATO's Operation Resolute Support mission in the country.
The slain individuals weren't immediately identified, pending notification of their family members, the release states. The injured service member and civilians are listed in stable condition, it states.
An investigation into the attack is underway, according to NATO. The incident occurred while the Americans were training Afghan security forces near a coalition base, it reported.
The unidentified assailant was killed, the release states. The individual was wearing an Afghan army uniform when he opened fire on the coalition forces around 11 a.m. local time, the website al-Jazeera reported.
"Anytime we lose a member of our team, it is deeply painful," Gen. John W. Nicholson, commander of U.S. Forces-Afghanistan and Operation Resolute Support, said in a statement.
"Our sympathies go out to the families, loved ones and the units of those involved in this incident," he added. "To those who continue to target coalition forces, ANDSF, and Afghan civilians, RS and USFOR-A will continue to pursue our Train, Advise, and Assist mission to help our partners create a better Afghanistan."
The death marks the 10th fatality of a coalition member in Afghanistan in 2016 -- and the eighth of an American service member this year, according to the website icasualties.org, which tracks the information based on public records.
Hundreds of coalition members have died or been wounded in so-called insider attacks in the country over the past several years, by some estimates.
At least 150 troops since 2008 have been killed in insider attacks -- and in 2012 the incidents accounted for as many as 15 percent of coalition deaths, according to an analysis by the online The Long War Journal.