Officials are investigating after an Army sergeant died in a non-combat incident.
U.S. and Iraqi commanders warn that victories over ISIS could be easily undercut by a large-scale withdrawal.
Mattis said that allied countries of origin should take custody of ISIS foreign fighters captured in Syria.
Keeping U.S. forces in areas claimed by the Assad regime inherently increases the probability of more clashes.
An IG report on the anti-ISIS mission cited reports that the militias, known as the PMF, had obtained nine of the tanks.
Air Force Col. John Thomas said that El Shafee Elsheikh and Alexanda Amon Kotey were captured in January in eastern Syria.
Turkey wants no confrontation with U.S. troops, but said they risk being caught up in clashes if they wear YPG uniforms.
Syria's Kurdish militia is growing frustrated with its patron, the United States.
President Trump signed an executive order to keep the detention facility at Guantanamo Bay, Cuba open.
Turkey renewed threats over the weekend to attack a town in northern Syria where U.S. troops maintain an outpost.