The report to a U.N. Security Council committee monitoring sanctions against the terrorist group did not say where the Bush administration believes such an attack might be launched.
But the United States said it believes that despite recent setbacks, "al Qaeda maintains the ability to inflict significant casualties in the United States with little or no warning."
"The al Qaeda network will remain for the foreseeable future the most immediate and serious terrorism threat facing the United States," the report said. "Al Qaeda will continue to favor spectacular attacks but also may seek softer targets of opportunity, such as banks, shopping malls, supermarkets, and places of recreation and entertainment."
The report said the terrorist organization "will continue its efforts to acquire and develop biological, chemical, radiological, and nuclear (CBRN) weapons."
"We judge that there is a high probability that al Qaeda will attempt an attack using a CBRN weapon within the next two years," it said.
A radiological weapon is a so-called "dirty bomb," which uses traditional explosives to disperse radioactivity. Such bombs could use lower-grade radioactive material, which can be more easily produced or obtained than the high-grade uranium and plutonium used for nuclear weapons.
Defense Secretary Donald H. Rumsfeld said last year that searches of more than 40 sites in Afghanistan used by al Qaeda yielded documents, diagrams and material that showed "an appetite for weapons of mass destruction." But it did not appear al Qaeda had succeeded in making such weapons before the U.S.-led military campaign began in October 2001.
The report said FBI investigations since the Sept. 11 terrorist attacks "have revealed an extensive and widespread militant Islamic presence in the United States."
"We strongly suspect that several hundred of these extremists are linked to al Qaeda," it said.
The U.S. report also noted that "there are hundreds of ongoing counter-terrorism investigations in the United States directly associated with al Qaeda," primarily on the east and west coasts and in the southwest.
"Identifying and neutralizing these sleeper cells remains our most serious intelligence and law enforcement challenge," the report said.
The activities of the groups identified by the United States center on fund raising, recruitment and training, but "one or more groups or individuals could be used by al Qaeda to carry out operations in the United States or could decide to act independently," it said.
"Al Qaeda most likely will use the same tactics that were successful on Sept. 11 in carrying out any future attack in the United States, including efforts by cell members to avoid drawing attention to themselves and to minimize contact with militant Islamic groups and mosques in the United States. They will also maintain strict operational and communications security," the report said.
In the past year, al Qaeda and its affiliates have been linked to a number of attacks: October's Bali nightclub bombing, the attack on a hotel and the attempt to shoot down a jetliner in Kenya in November, and the bombings at residential compounds in Riyadh, Saudi Arabia, last month.
Meanwhile, the effort by an alliance of governments to destroy the terror network is meeting both success and challenges.
Several high-ranking al Qaeda suspects — Khalid Sheik Mohammed, Ramsi Binalshibh, Abu Zubaydah — have been captured, and others killed.
An Australian TV report says police and intelligence agencies last year foiled plans by the regional terror group Jemaah Islamiyah to set up a network in Australia.
But al Qaeda founder Osama bin Laden's status is still unknown.
And at home, House Democrats say the Homeland Security Department is ill-equipped to analyze the bioterror threat to the nation, which brings into question a $6 billion administration-backed plan to stockpile antidotes.
A department says officials are moving quickly to rectify staffing and space shortcomings.