Three US soldiers were killed in Afghanistan when a man in an Afghan army uniform turned his weapon on them in the eastern province of Paktika.
So-called insider attacks by Afghan soldiers on their NATO-force allies have become an increasing problem over the past year.
Last year, a surge in such attacks prompted NATO to temporarily curtail some joint operations with Afghan government forces.
Insider attacks accounted for one in every five combat deaths suffered by NATO-led forces in Afghanistan and 16% of all US combat casualties, according to 2012 data.
Separately, an Italian soldier was killed and three were wounded when a child threw a grenade at a NATO convoy in the western province of Farah.
The four deaths bring to 16 the number of international troops killed in Afghanistan this month.
On Thursday, seven Georgian soldiers were killed in a suicide car-bomb attack in the southern province of Helmand.