Testifying before a House of Representatives subcommittee, the US State Department's counter-terrorism coordinator Cofer Black, however, on Thursday said the outlawed organisation still remained a "potent force."
Black depicted a decentralised organisation in which most seasoned leaders have been killed or captured.
The top leaders have been replaced by inexperienced people prone to mistakes, such as a November bombing in Saudi Arabia that killed mostly Muslims, Black said.
He also told the House International Relations subcommittee that there was "mounting evidence of al-Qaida's links to the 11 March bombing in Madrid that killed 191 people."
Countering accusations that the Bush administration ignored the al-Qaida threat during its initial months in office, Black said the administration has been "very successful to date" in fighting terrorism.
"The al-Qaida organisation we engaged before 9/11 and at 9/11 has been put under catastrophic stress," he said.
Usama bin Ladin's network "has been gravely wounded and forced to evolve in new ways to survive," Black said, by "trying to co-opt the missions of other terrorist groups" to attack US interests.
Black said there are indications that al-Qaida's "ideology is spreading well beyond the Middle East."
Black said there is greater cooperation between al-Qaida and smaller Islamist groups and more localised organisations.
But, with al-Qaida's ranks of leaders mostly killed, captured or on the run, Black said the group was "left with far fewer people that know how to do major operations effectively, securely."