Around 60 Islamic State fighters had been deployed in several cities in Europe shortly before the Paris attacks in November last year, with plans of carrying out major attacks on five cities, a senior European counterterrorism source told CNN.

The source reportedly said that Western security agencies had received intelligence on the extremist group's targets in Europe. The intelligence indicated that the Islamic State group, also known as ISIS, had planned attacks in Paris, London, Berlin and a major population center in Belgium, though there was no indication that the attacks were to take place simultaneously, the source told CNN. It also indicted that Abu Mohammad al-Adnani, the leader of ISIS’ external operations, was the mastermind behind the plan, the source added.

"In terms of ambition, it also just pointed towards something we already knew. ISIS had hardly made it a secret it sought to target Europe," the senior European counterterrorism source reportedly said.

In 2014, ISIS made several threats in a series of audio messages warning of major attacks on several European countries. The threats also reportedly made specific references to France and Belgium. Meanwhile, in January 2015, the Belgian police thwarted a major ISIS plot allegedly coordinated by Abdelhamid Abaaoud, the ringleader of the Nov. 13, 2015, Paris attacks that killed 130 people.

Western intelligence agencies suspect that Adnani is at the center of planning an international attack. "As far as we're concerned, he is top of our target list," the source told CNN.

The source also said that lack of specific intelligence on details about the Paris plot has caused tensions because it rarely happens that a terrorist plot completely goes under the radar.

"The worry has been that if, say, 20 were involved in the Paris attacks, there might be 40 or so still out there," the source said.

Since the Paris attacks, French and Belgian police have conducted several raids to find ISIS supporters planning attacks in Europe. Most of those involved in the Paris attacks were from Belgium.

Several ISIS fighters are reportedly believed to have left Syria and entered the U.K. along with Paris attacks’ mastermind Abaaoud. At least 90 ISIS militants are reported to have entered Europe last year. A Western counterterrorism official told the Wall Street Journal in December that members of Abaaoud's network traveled through Greece, Bulgaria and Romania, before moving out to other countries, including Britain, Reuters reported.