The former head of the CIA has warned that the Islamic State (ISIS) may launch another terrorist attack in Europe similar to last Friday's Paris massacre, saying this could already be underway and could be difficult to counter.

John Brennan said Europe is currently unable to foil future tragedies because its security services are being severely "overstretched."

"I would anticipate that this [Paris attack] is not the only operation that ISIL [another acronym for ISIS] has in the pipeline," said Brennan, according to the Daily Express.

"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."

"We're in a situation where the services are overrun. They expect something to happen, but don't know where and you have to see how much stress they are under," he said.

Brennan's chilling warning comes as France, Belgium and others reportedly stepped up their efforts in a manhunt for the suspects of Friday's attack that killed 129 people and wounded more than 300.

Three connected groups of militants targeted a music concert, a football match and several restaurants, according to news reports.

The offensive, according to Brennan, appeared to have been planned thoroughly "in terms of making sure they had the operatives, the weapons, the explosives, the suicide belts."

Michael Morell, former acting director and deputy director of the CIA, said Brennan's concerns are well-founded.

"The attack in Paris was the first manifestation of an effort that ISIS made to put together an attack capability in Europe—an effort that they began less than a year ago. The head of the U.K.'s domestic security agency recently warned that ISIS is planning mass casualty attacks in Britain. His concerns are well founded. We will not be far behind,'' said Morell, according to Time.

"Over time, if not significantly degraded, the ISIS threat to the [U.S.] will become a direct one—that is, an ISIS ability to plan and direct attacks on the [U.S.] from the group's safe haven in Iraq and Syria— just like what the group did in Paris last week,'' he added.

Morell released a book earlier this year on the war against international terrorists. The book, "The Great War of Our Time: CIA's Fight Against Terrorism — From al Qaida to ISIS," warns against the types of attacks that occurred in the Sinai and Paris.

Paris officials have identified three men as being responsible for Friday's carnage. Two are known to European and US intelligence services even before the killings took place, said the Express

The other, a Belgian man, named Abdelhamid Abaaoud is believed to be the mastermind behind the event, said the report.