US Secretary of Defense Ashton Carter said on Wednesday airstrikes by Russia in Syria in support of Assad is like pouring gasoline on fire.
Fighting ISIS without following up on the political transition in Syria only further endangers internal war in Syria, he added.
US officials are concerned that Russia will target opposition groups, The Hill wrote, adding supporting Assad to remain in power will inflame and prolong the 4-year civil war.
Russian airstrikes were in areas where ISIS was not present and other groups were active there, Carter added.