Al-Qaeda's operating capabilities are at their strongest level since the 11 September 2001 attacks, according to leaks of a US intelligence report.
Al-Qaeda has regained its strength despite prolonged military action
It suggests the network has rebuilt itself despite a six-year campaign to dismantle it.
The classified report identifies Pakistan's western tribal areas as the group's safe haven, and examines threats posed to the US and its allies.
It was compiled by the National Counterterrorism Center (NCTC).
The five-page threat assessment document, Al-Qaeda Better Positioned to Strike the West, was to be discussed at the White House on Thursday.
Al-Qaeda is "considerably operationally stronger than a year ago" and has "regrouped to an extent not seen since 2001," an official said, paraphrasing the report.
"They are showing greater and greater ability to plan attacks in Europe and the United States."
Of particular concern to the NCTC, reports BBC security correspondent Frank Gardner, is the possibility of a major al-Qaeda attack on America being carried out by Europe-based jihadists, particularly Britons, who are able to enter both Pakistan and the US with equal ease.
Counter-terrorism officials caution that just because there has been no repeat of 9/11 does not mean that al-Qaeda will give up trying.
The latest thinking of US intelligence was reflected in the testimony on Wednesday - before the House Armed Services Committee - of senior officials.
The CIA's deputy director for intelligence, John Kringen, told the hearing that al-Qaeda had created a safe haven in "the ungoverned spaces of Pakistan" - a reference to the rugged, lawless terrain of the Pakistani-Afghan border.
"We see more training. We see more money. We see more communications. We see that activity rising," he said.