Three U.S. service members and a contractor were killed on Monday by an improvised explosive device in Afghanistan.
NATO's Afghanistan mission said in a statement that three others were injured and are receiving medical care.
The incident occurred near the Bagram Air Base.
The names of the service members are being withheld for 24 hours based on Department of Defense policy.
The attack comes as the U.S. and the Taliban have been holding peace negotiations.
A fifth round of talks ended last month in Qatar, and another conference between the groups is set for this month.
Two U.S. soldiers were killed in Afghanistan's Kunduz Province in March.
The Associated Press reported that the Taliban has claimed responsibility for attack.
--Updated at 3:38 p.m.