CIA Director John Brennan responds to a question as he speaks at the Global Security Forum 2015, Monday, Nov. 16, 2015, at the Center for Strategic and International Studies (CSIS) in Washington. (AP Photo)
CIA director John Brennan warned Monday that the attacks in Paris were likely not a "one off event" and that he expects the Islamic State group has more operations in the pipeline.
"Security and intelligence services right now are working feverishly to see what else they can do in terms of uncovering it," he said at a Washington think tank.
The CIA chief said Friday night's attacks by gunmen in suicide vests in the heart of the French capital were carefully planned and executed.
"This was not something done in a matter of days. This is something that was carefully and deliberately planned over the course of several months in terms of whether they had the operatives, the weapons, explosives, suicide belts.
"I would anticipate that this is not the only operation ISIL has in the pipeline," he said, using an alternate acronym for ISIS, the militant group that has seized large areas of Syria and Iraq.
At least 129 people were killed in the attacks carried out by at least eight gunmen nearly simultaneously at a stadium, a concert hall and restaurants as throngs of Parisians were enjoying a night out.
The attacks were the latest in a burst of horrific attacks attributed to ISIS -- following twin suicide bombings that killed 44 people in Beirut on Thursday.
"They are looking abroad to have these spectacular attacks to further their narrative that the caliphate is growing and is successful," Brennan said.
Intelligence services were aware that ISIS was planning attacks outside their strongholds in Iraq and Syria, particularly targeting Europe, Brennan said.
"I can tell you it was not a surprise that this attack was carried out, from a standpoint that we did have strategic warning, we knew plans and plotting by ISIL was underway, looking particularly at Europe as a venue for carrying out these attacks," he said.
But the large numbers of individuals who have gone to Syria and Iraq and then returned to Europe, he said, has strained the capacity of European intelligence services to monitor them all.
The fact that such a complex plot was carried out without being detected also shows ISIS has "gone to school on what they need to do in order to keep their activities concealed from the authorities," he said.
Brennan made a pitch for reviewing curbs placed in recent years on the intelligence services' surveillance capabilities, saying "unauthorized disclosures" and "handwringing" had made international efforts to track down terrorists "much more challenging."
The Paris attacks show ISIS is shifting its sights to the West, raising questions about a US-led strategy that has focused on air strikes on its strongholds in Syria.
Brennan, however, argued that containing the group in the region remains the most effective strategy, and said there have been notable successes in disrupting the flow of people and material needed to carry out attacks.
Likewise, he cautioned that closing borders either in Europe or the United States was inconsistent with western values and "not sustainable."
"We don't want to have these terrorists succeed in taking away the freedoms and liberties that we pride ourselves on, whether it be here in the United States or in Europe."
But he said no country was "immune from ISIL's touch."