U.S. and Afghan forces are seeking to recapture areas that were once under American control.
The Islamic State and Taliban have been battling each other in a remote corner of Afghanistan.
Afghanistan’s military said Afghan helicopters attacked a Taliban stronghold. Others say it was a religious school.
Trump issues order supporting ban on many transgender troops, defers to Pentagon on new restrictions
The plan is expected to be immediately challenged in federal court.
A Pentagon watchdog previously highlighted misuse of government charge cards.
'There is no reason for North Korea to view them as a provocation,' a Pentagon official said.
The deadly attack underscores the danger of dispersing small teams across a vast continent.
Marine Gen. Joseph F. Dunford Jr., the chairman of the Joint Chiefs of Staff, will soon endorse the investigation, Mattis said.
'The durability of our relations with these countries ... goes beyond personalities,' Mattis said.
"These networks need to be identified and destroyed,” said Gen. John W. Nicholson Jr., the top U.S. general in Afghanistan.