The Islamic State (ISIS) terrorist organisation remains as dangerous as ever and is likely to intensify its global terror campaign even as it continues to lose territory in the Middle East.

Testifying before Congress on Thursday, CIA Director John Brennan revealed that ISIS can count on a "large cadre of Western fighters" who could launch surprise attacks in the U.S. and other Western countries, CNN reports.

"Unfortunately, despite all our progress against ISIL on the battlefield and in the financial realm, our efforts have not reduced the group's terrorism capability and global reach," Brennan told the Senate Intelligence Committee, using another acronym for the terrorist group.

"The resources needed for terrorism are very modest, and the group would have to suffer even heavier losses of territory, manpower and money for its terrorist capacity to decline significantly," he added.

"In fact, as the pressure mounts on ISIL, we judge that it will intensify its global terror campaign to maintain its dominance of the global terrorism agenda," the CIA chief warned.

The group is already preparing more attacks by its covert fighters who may have blended with the refugees streaming into Western nations, he said.

"We judge that ISIL is training and attempting to deploy operatives for further attacks," Brennan said. "ISIL has a large cadre of Western fighters who could potentially serve as operatives for attacks in the West. And the group is probably exploring a variety of means for infiltrating operatives into the West, including refugee flows, smuggling routes, and legitimate methods of travel."

The CIA director spoke just four days after the terrorist attack in Orlando that left 49 people dead.

Brennan confirmed that although the Orlando shooter, Omar Mateen, had "no direct links" to ISIS, he was inspired by the terrorist group.

"We have not been able to uncover any direct link between that individual, Mateen, and a foreign terrorist organisation. But that inspiration can lead someone to embark on this path of destruction," he said.

Brennan pointed out that lone wolf attackers like Mateen, who are inspired by but not under the direct control of terror groups, represent "an exceptionally challenging issue for the intelligence community."

The CIA director said technology and communication companies could help in the fight against ISIS by collaborating with law enforcement agencies, noting that encryption is allowing ISIS and its followers to communicate clandestinely.

He revealed that ISIS preferred social media propaganda platforms are Twitter, Telegram and Tumbler.

"They're taking advantage of the liberties that we've fought so hard to defend," Brennan said.