The Islamic State of Iraq and the Levant (ISIL, also known as ISIS) remains "formidable" and "resilient" despite efforts to defeat it militarily, the director of the CIA has warned, adding that the group has tens of thousands of fighters around the world - far more than al-Qaeda had at its height.

John Brennan told the US Senate intelligence committee on Thursday that ISIL was losing ground in Syria and Iraq but the group retained its ability to conduct attacks around the world.

"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," he said.

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"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."

Brennan said that ISIL has a "large cadre of Western fighters who could potentially serve as operatives for attacks in the West".

His comments came a few days after an attack at a gay nightclub in the US city of Orlando  left 49 people dead - the worst mass shooting in US history.

The shooter, Omar Mateen, had called 911 during the attack at the Pulse nightclub early on Sunday to express his allegiance to ISIL.

But Brennan said the CIA has found no connection between the Orlando shooter and a foreign "terrorist" organisation.

He added that the CIA was sharing intelligence with the FBI to help identify potential lone-wolf attackers, but the agency's responsibility was to gather information about operations overseas.

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"This is a global challenge," Brennan said. "The number of ISIL fighters now far exceeds what al-Qaeda had at its height."

He said there were 18,000 to 22,000 ISIL fighters in Syria and Iraq - down from about 33,000 last year.

The branch in Libya, with between 5,000 and 8,000 fighters, was likely the most advanced and most dangerous, but ISIL was trying to increase its influence in Africa and to plot attacks in the region and in Europe, Brennan said.

He said Boko Haram was now the ISIL branch in West Africa and had several thousand fighters.

Source: Agencies