The Islamic State "has developed an external operations agenda it is now implementing with lethal effect" and will continue to try to attack Western targets as the military campaign against the extremist group squeezes its strongholds in Iraq and Syria, CIA Director John Brennan said Monday.

Meanwhile, the terrorists are getting better at hiding their plans from global intelligence agencies, exploiting new technologies and government policies put in place in the wake of leaks by former National Security Agency employee Edward Snowden to conceal their operations, he said, noting that officials would have to quickly adapt to thwart the next attack.

"I do hope that this is going to be a wake-up call," Brennan said during a talk at the Center for Strategic and International Studies' annual Global Security Forum. "This is not the only operation that [the Islamic State] has in the pipeline."

Though Friday's terrorist attacks in Paris that killed 129 people and injured more than 350 appeared to have been planned weeks or months in advance and were preceded by warnings that the Islamic State was planning to strike in Europe or the United States, the plotters were able to avoid detection.

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