Defense Secretary Ash Carter is criticizing Russia for bombing U.S.-backed rebel forces in southern Syria that he says were fighting the Islamic State group.

In remarks to reporters at the Pentagon on Friday, Carter calls the Russian actions "problematic" and suggests the Russian military misused a channel of communication with the U.S. military that is intended to prevent unsafe U.S. and Russian air operations over Syria.

Carter says that if the Russians intentionally attacked U.S.-supported Syrian rebels, then they were contradicting their stated aim of fighting the Islamic State. And if the attack was accidental, then it speaks poorly of the quality of information the Russians use to conduct their airstrikes.

Carter also says that in Iraq, government forces have gained control of "a portion" of IS-held Fallujah.