The efforts of the US-led coalition fighting so-called Islamic State (IS) have failed to reduce its ability to carry out militant attacks, CIA Director John Brennan says.
Mr Brennan told a hearing the group remained "formidable" despite territorial losses.
He estimated that IS now has more fighters than al-Qaeda when that group was at its strongest.
As many as 22,000 IS fighters are operating in Iraq and Syria, he said.
Mr Brennan made the comments to the Senate intelligence committee in an update on the threat from extremists.
"Unfortunately, despite all our progress against Isil [Islamic State] on the battlefield and in the financial realm, our efforts have not reduced the group's terrorism capability and global reach," he said.
The CIA was particularly concerned about the growth of Libya as an IS base, he said.
While he said the US-led coalition had made progress against IS, the group has "a large cadre of Western fighters who could potentially serve as operatives for attacks in the West".
"To compensate for territorial losses, Isil will probably rely more on guerrilla tactics, including high-profile attacks outside territory it holds," he warned.
His appearance came days after the attack on a gay nightclub in Florida. The gunman, Omar Mateen, had pledged allegiance to IS.
But Mr Brennan told the hearing the CIA had not uncovered any direct link between Mateen and foreign militant groups.
