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Updated at 5:35 p.m. ET

An Afghan soldier opened fire on his U.S. counterparts on Saturday, killing three Americans and wounding at least one other. A spokesman for the provincial governor of Nangarhar, in eastern Afghanistan, confirmed that the incident occurred during an operation in the district of Achin, on the Pakistan border.

"In (a) joint US-Afghan military operation ... American troops were killed when an Afghan member of army commando opened fire on them," Attaullah Khogyani told Al-Jazeera earlier in the day. "The Afghan commando was also killed in counterattack."

The Associated Press reports that the Taliban has claimed responsibility for the shooting, noting that the militant group released a statement saying one of its loyalists had joined the Afghan military "just to attack foreign forces."

According to the wire service, the Pentagon says the U.S. military is "aware of an incident in eastern Afghanistan."

The Pentagon confirmed the number of deaths in a statement Saturday evening:

"Three U.S. soldiers were killed in eastern Afghanistan today. One U.S. soldier was wounded and has been evacuated for medical treatment. Next of kin notification is underway.

"This incident is under investigation. Additional information will be released as appropriate."

Vice President Pence addressed the incident during an appearance Saturday.

"On my way here, I was informed that U.S. service members were killed and wounded at an attack in Afghanistan," Pence said Saturday at a speech in Milwaukee. "The president and I have been briefed. The details of this attack will be forthcoming. But suffice it to say, when heroes fall, Americans grieve. And our thoughts and prayers are with the families of these American heroes."

As Jennifer Glasse reports for NPR's Newscast unit, Achin has been a major base for Islamic State militants.

"U.S. special forces have been working on the ground there with Afghan forces," Jennifer notes, "and in April, the United States dropped its [most powerful] non-nuclear bomb in the area to destroy tunnels being used by ISIS fighters and killed dozens of them, according to locals in the area."

She adds that shootings like the one that occurred Saturday "are called insider attacks or green on blue with Afghans wearing green shooting Americans and NATO soldiers."

And it's not the first time this year that one has occurred, as NBC News explains: "The attack follows one in March in which three American soldiers were shot and wounded by an Afghan soldier on a base in Helmand province, officials said. That Afghan soldier was also killed following the incident."

Reuters reports that shortly before Saturday's attack, at least three members of the Afghan security forces were killed by U.S. aircraft during an overnight raid in Helmand.

"We would like to express our deepest condolences to the families of the [Afghan Border Police] members affected by this unfortunate incident," a U.S. military spokesman said in a statement, according to the news service.

NPR's Amy Held notes that while the U.S. officially withdrew from Afghanistan in 2014, roughly 8,400 troops remain in the country and the Trump administration has been considering requests to increase those troop levels.

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At Least 3 US Soldiers Killed In 'Insider Attack' In Afghanistan - NPR