The rogue soldier, a commando who headed an 11-man unit, was also killed in return fire.
"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".
During such assaults, members of the Afghan security forces are known to target USA and other North Atlantic Treaty Organisation soldiers. The area has been cordoned off by United States troops.
Kazem, an Afghan army soldier, said that such attacks are sometimes planned by the Taliban, but most of the time it is caused by minor disagreements between the two sides. The White House said Army Reserve Major General Rick Waddell provided updates.
"When heroes fall Americans grieve", Pence added. "And our thoughts and prayers are with the families of these American heroes".
"An investigation is being conducted at this time to determine the specific circumstances that led to this incident", USA military officials said in a statement. Although military officials say the group is far smaller than it was at its height in 2015, an estimated 600 to 800 militants, located mainly in remote mountainous areas, continue to pose an ample threat to USA and Afghan troops.
North Atlantic Treaty Organisation troops have adopted special security measures in recent years to try to counter the threat.
The latest attack comes as US President Donald Trump's administration is considering sending more troops to Afghanistan.
Omar Zawak, spokesman for the governor of Helmand, said the deaths occurred when a United States aircraft fired on Afghan police, the statement said.
Local news media reported at least two deaths, while Omar Zwak, a spokesman for Helmand's governor, said that three officers were killed and two were wounded.
The Afghan conflict is the longest in American history.
Taliban insurgents have made widespread gains in Helmand, seizing numerous province's districts and threatening the provincial capital of Lashkar Gah.
There are now 8,400 American troops in Afghanistan training, advising and assisting the Afghanistan army.
American troops, including special forces, also carry out separate counterterrorism missions.
Three American servicemembers were killed in an attack in Nangarhar province in eastern Afghanistan on Saturday, June 10, 2017.