"That's going to be a tough challenge to separate them, if we were to take a decision to strike," said Dempsey, adding that separating the two groups is important because the government needs to reach out to the Sunnis to move the country forward.
Dempsey also confirmed that Iran is taking a more overt role in Iraq, flying drones over the country and providing military equipment to the Iraqis. He said the U.S. won't coordinate any military action with Tehran and doesn't need to communicate with Iranians in Iraq to avoid conficting flights because that is the Iraqis' responsibility.
In other developments Thursday, Saudi Arabia massed tens of thousands of troops along its border with Iraq. A U.S. official said the decision comes as Saudi Arabia moved to protect itself in case the insurgency moves south.
Associated Press broadcast journalist Sagar Meghani and writer Donna Cassata contributed to this report.
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