An Afghan civilian employed on a NATO base has shot dead three NATO soldiers - on the same day three others were killed by an Afghan policeman.
A spokesman says the shooter was authorised to be on the base.
He said he had no information on how the civilian got hold of a weapon at a joint Afghan-NATO base in southern Afghanistan.
Also on Friday, an Afghan police officer opened fire on four United States soldiers he had invited for a meal, killing three of them.
Afghan officials have said that the three were special forces members.