Defense Secretary Ash Carter is casting doubt on prospects for a military partnership with Russia to combat the Islamic State inside Syria.

At a Pentagon news conference Monday, Carter says the Russians are "a long way" from aligning their military focus with that of the United States.

Carter says the Russians should be combatting the Islamic State rather than the moderate opposition that must be part of a political transition that could end Syria's civil war.

Secretary of State John Kerry has been talking to senior Russian officials about a proposal in which the U.S. would share intelligence and targeting information with the Russians. In exchange Moscow would use its influence with the Syrian regime to effectively ground the Syrian air force.