The militant group acknowledged the attack but did not claim responsibility. Western officials say most insider attacks stem from personal grudges and cultural misunderstandings rather than insurgent plots.
The incident is being investigated.
It was not immediately clear what became of the shooter.
He urged both groups to come to the negotiating table, adding, the USA would be willing to help facilitate the discussions.
But a police officer told news agency AFP the shooting happened at the airport in Tarinkot, the capital of Uruzgan province - although they were unable to confirm this detail with any North Atlantic Treaty Organisation officials.
Tropical Depression Two forms over the Atlantic this morning
A tropical wave heading for the Caribbean is now a tropical depression and is headed for the Lesser Antilles. The first National Hurricane Center advisories for Tropical Depression 2 will begin at 11 a.m.
Maciel's task force was deployed in support of the 1st Security Force Assistance Brigade, a newly established unit that is focused on advising and bolstering local troops and is seen as critical to America's strategy in Afghanistan.
To date, 49 USA troops have died in Afghanistan since January 1 2015, when operations in the country formally transitioned from combat operations to a mission of support and advisory assistance to the Afghan National Security Forces, who continue to fend off Taliban insurgents on the ground.
U.S. Secretary of State Mike Pompeo used an unannounced trip Monday to Afghanistan to step up the Trump administration's calls for peace talks between the Afghan government and the Taliban.
The names of the service members will not be released until their next of kin were notified.
However, details of the attack's location were not revealed, reports The Washington Post.
The latest incident comes as Lieutenant General Scott Miller is set to become the next commander of USA and North Atlantic Treaty Organisation forces in Afghanistan.