The new assessment, to be presented to Congress next week, says al Qaeda represents the gravest danger even though a number of Muslim extremist groups unconnected to the terror organization also are committed to "jihad," or holy war, against the United States, Israel and other countries.
The report comes amid new jitters about potential terrorist attacks timed to coincide with the Hajj, the annual pilgrimage Muslims make to Mecca. The holy period begins Sunday.
Despite an increase in intelligence traffic and the FBI report, the White House said Thursday there was no change in the terror alert status, reports CBS News Correspondent John Roberts.
It now stands at yellow, or elevated, which is the middle tier in a five-color system.
The FBI warning also comes as the United States amasses military power for a possible war with Iraq to force disarmament. Secretary of State Colin Powell presented evidence to the United Nations on Wednesday to buttress the U.S. position, including assertions that al Qaeda has some ties to Iraq.
The FBI's central conclusion is that "al Qaeda led by Osama bin Laden remains, for the foreseeable future, the most serious threat against the United States," a government official familiar with a declassified version of the assessment said Wednesday.
Other terrorist groups provide "varying degrees of support" for al Qaeda but are less likely to strike within the United States, the report adds.
"We remain concerned about the continued al Qaeda activity overseas as well as al Qaeda sympathizers here in the United States," Gordon Johndroe, spokesman for the Homeland Security Department, said. "Should additional information and analysis develop requiring the threat level to be raised, we will keep the American public informed as always."
The government official declined to discuss whether the FBI assessment describes a greater danger of terrorism if the United States goes to war with Iraq.
But Germany's top counter-terrorism official, Interior Minister Otto Schily, told reporters Wednesday that such a war — which his government opposes — would inflame Muslim extremists worldwide and increase the terror threat.
"If a war takes place, the emotions will intensify," said Schily, who met this week with Attorney General John Ashcroft and Homeland Security Secretary Tom Ridge. "It's a matter of concern. You can't exclude repercussions."
The FBI assessment, most of which is classified, will be delivered to members of the House and Senate intelligence committees next week by FBI Director Robert Mueller. Originally promised in 2000, a version was completed just a day before the Sept. 11 terror attacks and has since been rewritten amid criticism from Capitol Hill for the delay.
The report cites various U.S. intelligence sources as evidence that al Qaeda operatives around the world continue to discuss large-scale strikes against the United States. Al Qaeda is actively attempting to acquire chemical, biological, radiological and even nuclear weapons, the report adds.
The organization remains viable despite being driven from its refuge in Afghanistan, the report says. It cites the October nightclub bombing in Bali, Indonesia, that killed nearly 200 and November attacks on a resort and airliner in Kenya as evidence the network can still inflict great damage.
The Kenya attacks occurred nearly simultaneously. A vehicle packed with explosives plowed into a hotel, killing 15 people. Minutes earlier, unidentified assailants fired two missiles at an Arkia Airlines Boeing 757, narrowly missing the charter aircraft as it took off from Mombasa airport with Israeli tourists returning to Tel Aviv.
Schily told reporters in Washington on Wednesday that the "strength of al Qaeda groups is as high as it was before Sept. 11, maybe also a little bit more than before."
"The threat has dimensions that are really dangerous," he said.
The FBI assessment concludes that al Qaeda or its sympathizers are mostly likely to mount smaller-scale attacks, perhaps on an individual basis, and that so-called "soft targets" such as the Bali nightclub could be hit because they are difficult to protect.
The assessment also details actions the FBI is taking to improve its counter-terrorism efforts, such as enhancing its intelligence analysis abilities, focusing more agents on fighting terrorism and upgrading its computer capabilities.