Three American troops were wounded Sunday when an Afghan soldier opened fire in southern Helmand province, officials said, in the first known "insider attack" on international forces this year.
No insurgent group has so far claimed responsibility for the attack in Camp Antonik in Washer district, which highlights long-simmering tensions between Afghan and foreign forces.
"Three US soldiers were wounded this afternoon when an Afghan soldier opened fire on them at a base in Helmand province. Coalition security forces on the base killed the soldier to end the attack," a spokesman for US forces in Afghanistan told AFP.
"The US soldiers are receiving medical treatment at this time and we will release more information when available."
An Afghan soldier was also killed in the shootout, provincial spokesman Omar Zwak told AFP.
The Pentagon has said it would deploy some 300 US Marines this spring to Helmand province, where American forces engaged in heated combat until they pulled out in 2014.
The Marines will head to the poppy-growing province this spring to assist a NATO-led mission to train Afghan forces, in the latest sign that foreign forces are increasingly being drawn back into the worsening conflict.
So-called insider attacks -- when Afghan soldiers and police turn their guns on their colleagues or on international troops -- have been a major problem during the more than 15-year-long war.
In May last year, gunmen wearing Afghan military uniforms shot dead two Romanian soldiers in neighbouring Kandahar province.
Western officials say that most such attacks stem from personal grudges and cultural misunderstandings rather than insurgent plots.
The killings have bred fierce mistrust between local and foreign forces even as the rate of such incidents has dropped in recent years.
The Afghan military, which has been built from scratch since the fall of the Taliban regime in 2001, has also struggled with insider attacks, high casualty rates and mass desertions.