In March, an Afghan soldier wounded three Americans in Helmand province.
The US Defense Department is conducting investigation, Pentagon said in a statement.
President Trump is monitoring the situation, a White House spokesman said.
Saturday's attack came just hours after an errant U.S. air strike killed and wounded at least six Afghan policemen in southern Helmand province, in the latest "friendly fire" incident.
The shootings occurred in the Achin District, where USA and Afghan troops have been carrying out a monthslong offensive against a local affiliate of ISIS, officials said.
US and Afghan troops have been battling militants in the Nangarhar province for months. The President and I have been briefed, the details of this attack are forthcoming. One of those wounded later died of injuries sustained in the attack, a US official tells CBS News.
Trump's Latest Move Sets Oil Prices Down
Brent for August settlement dropped as much as 90 cents, or 1.8 percent, to $49.73 a barrel on the London-based ICE Futures Europe exchange.
The operations are being conducted in coordination with the U.S. forces based in Afghanistan who are mainly carrying out airstrikes as part of a broader role granted to them by the Obama administration previous year.
Taliban claims responsibility for latest "green-on-blue attack" during joint operation in Achin district of Nangarhar. When heroes fall, Americans grieve.
The U.S. military is reportedly "aware" of the attack but has released no further information.
Insider or so-called "green-on-blue" attacks have been an ongoing threat to US military personnel in Afghanistan.
As Jennifer Glasse reports for NPR's Newscast unit, Achin has been a major base for Islamic State militants.
The U.S. has 8,400 troops deployed in Afghanistan to train and advise Afghan forces; most rarely participate in direct combat.
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.