Two American troops were killed and six others wounded when an Afghan soldier fired a machine gun into a group of other soldiers, both American and Afghan, the U.S. military confirmed Sunday.
The deaths come at a sensitive time in the 18-year-long war, with U.S. President Donald Trump eager to bring troops home and end the longest conflict in U.S. history.
The incident occurred late Saturday at a base in Sherzad district, in Nangarhar province, eastern Afghanistan.
"Current reports indicate an individual in Afghan uniform opened fire on the combined U.S. and Afghan force with a machine gun," U.S. Forces-Afghanistan spokesman Sonny Leggett said in a statement on Sunday.
Provincial Gov. Shah Mahmood Meyakhil said in an audio message to reporters that three Afghan commandos were wounded.
He said it was not immediately clear if the incident was a deliberate act by an "infiltrator" or an accident.
"It was not a clash between the forces. We are investigating," Meyakhil said.
Leggett also stressed the cause or motive behind the attack was not immediately known.
In a tweet, the U.S. Army's 7th Special Forces' airborne division said "several" of its soldiers had been killed or injured during combat operations in Afghanistan. No group immediately claimed responsibility for the incident.
Last year was the deadliest for U.S. forces in Afghanistan since combat operations officially ended at the end of 2014, highlighting the challenging security situation that persists.
Depending on how one qualifies combat deaths, about 20 American troops were killed in 2019. Since the U.S.-led invasion in October 2001, about 2,400 U.S. troops have been killed in combat in Afghanistan.
In December, Taliban infiltrators in the Afghan military killed nine Afghan soldiers in central Afghanistan. Meanwhile, in July, an Afghan soldier killed two U.S. troops as they visited an Afghan army base in Kandahar. Such insider attacks are sometimes called "green-on-blue" incidents.
The attack comes two weeks after another Afghan soldier shot and killed an influential Afghan army colonel while the latter was conducting a security assessment in Ghazni province.
U.S. special envoy Zalmay Khalilzad has been negotiating with the Taliban for around 18 months aiming for a deal that would see the Pentagon pull thousands of troops from Afghanistan.
Many are skeptical of the proposed deal and Trump himself declared talks "dead" in September amid continued Taliban violence. Negotiations have since resumed in Qatar.