The United States n talks about the necessity of change of the National security strategy.The CIA Director John Brennan warned on Monday that the attacks in Paris claimed by the extremist Islamic State movement were not a “one-off event” and that the militants may have similar operations ready to launch.

Foiling those plots, however, could prove difficult because Europe"s intelligence and security resources are severely stretched trying to keep track of the hundreds of European extremists who have returned home from fighting in Syria and Iraq.

"A lot of our partners right now in Europe are facing a lot of challenges in terms of the numbers of individuals who have traveled to Syria and Iraq and back again, and so their ability to monitor and survey these individuals is under strain,” Brennan said.

Brennan’s comment at a Washington policy institute came as France, Belgium and other countries intensified a manhunt for suspects in Friday’s attacks on a concert hall, sports stadium, restaurants and bars in Paris that killed 129 people.

U.S. intelligence still hasn’t confirmed that the Islamic State was responsible, said Brennan. But, he added, the Paris attacks and the suspected bombing of a Russian airliner in Egypt on Oct. 31 that killed all 224 passengers and crew aboard “bear the hallmarks” of the Islamist group.

The Islamic State, which threatened in a new video on Monday to attack in Washington, appears to have formed an external operations branch that may have readied follow-up strikes to the Paris attacks, he said.

“I would anticipate that this is not the only operation that ISIL has in the pipeline,” Brennan said, using an acronym for the Islamic State. “And security intelligence services right now in Europe and other places are working feverishly to see what else they can do in terms of uncovering it.”

Careful planning for the Paris strikes is believed to have taken place over several months “in terms of making sure they had the operatives, the weapons, the explosives, the suicide belts,” Brennan said.