Next of kin notification is underway.
"An investigation is underway to determine circumstances that led to the "unfortunate" incident", said the American military's public affairs office in Kabul. "We will release more information when appropriate".
"An Afghan army soldier opened fire and killed two American soldiers and injured two others", Attauallah Khogiani, spokesperson for the governor of Nangahar province, said. The Achin district in eastern Nangarhar province, where the attack took place, is also thought to be a stronghold of IS.
Zabihullah Mujahid, a spokesman for the Taliban, wrote in a WhatsApp message that the attacker was a Taliban infiltrator. "The president and I have been briefed". But suffice it to say, when heroes fall, Americans grieve. "And our thoughts and prayers are with the families of these American heroes", he added.
Insider attacks had frequently happened in the past, when an Afghan solider or policeman fires on foreign forces, causing casualties.
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The attack comes soon after a case of friendly fire against Afghan forces.
For example, the use of "guardian angels" has become standard in Afghanistan.
During such assaults, members of the Afghan security forces are known to target USA and other North Atlantic Treaty Organisation soldiers.
American troops have partnered with Afghan soldiers in raids against IS Khorasan, claiming the local offshoot of the jihadist group based in Iraq and Syria is steadily losing ground in Afghanistan. Once trained to conduct raids much like their US counterparts, the majority of Afghan commando units have turned into a backbone force for the fledgling Afghan army. The official asked not to be identified because the information had not yet been officially released and the USA military was still investigating the episode.
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