KABUL, Afghanistan — An Afghan working on an installation shared by Afghan and foreign forces shot to death three U.S. service members, raising to six the number of Americans killed by their Afghan partners in a single day, officials said Saturday.

The newly announced killings took place Friday, the same day that an Afghan policeman gunned down three U.S. Marines in a separate attack in southern Afghanistan.

Such assaults are on the rise and have heightened mistrust between foreign forces and the Afghan soldiers, police and others they are training and mentoring.