President Barack Obama signaled the U.S. has no immediate plans to escalate military operations against Islamic State extremists in Iraq or Syria, stressing the need to counter the group's advance while formulating a broader strategy to protect U.S. interests and allies.

Mr. Obama spoke on a day when Syrian opposition activists said the Sunni radical group had killed nearly 500 people since Sunday in the northeastern province of Raqqa, most of them Syrian troops captured on an air base seized by Islamic State fighters.

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