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".
An Afghan soldier opened fire on his USA counterparts on Saturday, killing three Americans and wounding at least one other.
A spokesperson for Nangarhar province, Attaullah Khogyani, told Al Jazeera that an Afghan "army commando" opened fire on four United States soldiers.
Spokesman Zabihullah Mujahid claimed on Twitter that four USA soldiers were killed in the attack. The U.S. military in a statement apologized for the deaths and said the incident was under investigation.
In March, three US soldiers were injured in Helmand Province after being fired upon by an Afghan soldier.
The so-called "friendly fire" incident occurred overnight, during a raid in Afghanistan's restive Helmand province, the United States military command in Kabul said in a statement. The official asked not to be identified because the information had not yet been officially released and the US military was still investigating the episode.
The attack comes as U.S. President Donald Trump's administration weighs an increase in troops in the war-torn country.
USA and Afghan troops have been battling militants in the Nangarhar province for months. "The president and I have been briefed".
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On Thursday, Iran's Foreign Minister Mohammad Javad Zarif criticized the language and rejected the condolences on Twitter .
Similar incidents have also plagued Afghan troops, depleting morale and causing mistrust within security ranks.
"Our thoughts and prayers are with the families of these American heroes", he said.
Taliban spokesman Zabihullah Mujahid in an email statement claimed responsibility for the attack, known as green-on-blue attacks, saying the attacking soldier was "an infiltrator" and had killed four USA soldiers.
The Taliban claimed responsibility for the attack.
Air raids by U.S. fighter jets have sharply increased in recent months, just as the White House considers requests from military commanders to send thousands more troops to Afghanistan. When heroes fall, Americans grieve.
Afghan border police were on a patrol in the volatile district of Nad Ali when they came under fire during a military operation around midnight on Friday.
There are 8,400 American troops in Afghanistan training, advising and assisting the Afghan military in its fight against the Taliban. "The soldier was also killed in the return fire", Khogyani told AFP.
"Sometimes the Taliban also kidnap a family member of the soldier and blackmail him to kill the USA troops to cause maximum number of casualties in such attacks".
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