US Director for Central Intelligence John Brennan said on Monday (16 November) Europe's proximity to Islamic State (IS) in Syria and Iraq has made it a prime target for acts of violence such as those perpetrated in Paris on Friday that left at least 132 people dead.

"A lot of our partners in Europe are facing a lot of challenge in terms of the numbers of individuals who have travelled to Syria and Iraq and back again so their ability to monitor and surveil these individuals is under strain," CIA Director John Brennan said during a forum on global security at the Center for Strategic and International Studies.

Gunmen and bombers attacked busy restaurants, bars and a concert hall at locations around Paris on Friday evening, killing dozens of people in what a shaken President Francois Hollande described as an unprecedented terrorist attack.

"When I think about what happened in Paris, clearly there was an effort that was under way for quite sometime that was fairly sophisticated because of the nature of the attacks in terms of their simultaneous nature," Brennan said.

The CIA director also made what appeared to be a veiled reference to Edward Snowden who famously leaked government documents about the US surveillance program.

"Because of a number of unauthorised disclosures and a lot of hand wringing over the government's role in the effort to try to uncover these terrorists, there have been some policy, and legal and other actions taken that make our ability, collectively, internationally to find these terrorists much more challenging," he said.

Brennan added that the US will continue to coordinate with France following what Hollande called an "act of war" by IS.