A spokesperson for Nangarhar province, Attaullah Khogyani, told Al Jazeera that an Afghan "army commando" opened fire on four U.S. soldiers.
A Pentagon statement confirmed that three U.S. soldiers had been killed in eastern Afghanistan and one wounded soldier had been evacuated for medical treatment.
The soldiers were killed in an apparent insider, or so-called "green-on-blue", attack by an Afghan ally soldier on USA service members.
In March an Afghan soldier was killed after he opened fire on his comrades - injuring three US soldiers.
It wasn't immediately clear if the shooting involved a jihadist sympathizer, a personal dispute or a cultural misunderstanding.
A United States warplane attacked a patrol of Afghan border police in the southern Helmand Province overnight, killing "several" and wounding a number of others.
US special operations forces have been fighting ISIS in the province alongside Afghan Special Security Forces. The area has been cordoned off by U.S. troops.
The US Defense Department is conducting investigation, Pentagon said in a statement. A spokesman for the provincial governor told the news service that the Afghan soldier was subsequently killed.
October 19, 2016 A gunman wearing an Afghan army uniform killed one American soldier and a civilian and wounded three more Americans in an attack in Kabul. The area has been cordoned off by USA troops.
Three US soldiers were killed and another was wounded Saturday in eastern Afghanistan's eastern Nangarhar province, the Pentagon said.
The attacker was killed after USA troops returned fire.
A similar insider attack wounded three US soldiers in March in Helman province, according to ABC News. Its spokesman Zabihullah Kujahid said online that the solider had been an "infiltrator" who had killed four American soldiers. The Afghan special forces units have quickly become the most reliable force in the more than 15-year-old war, American officials believe.
Khogyani said that the Afghan soldier had been killed during the attack. The larger of the two, known as Resolute Support, is focused on advising and assisting Afghan security forces with the goal of enabling them to independently protect the country's populace from a resilient Taliban.