The announcement came as US intelligence analysts reported increased communications among al-Qaeda cells around the world, which they said could be an indication that preparations for a new terrorist attack against the United States could be under way.
The Federal Bureau of Investigation has received information al-Qaeda leaders had discussed a possible plan to rent apartment units in the United States and plant explosives there, FBI spokeswoman Debra Weierman told AFP.
"We have no indication that this proposed plan went beyond the discussion stage," she said. "But in an abundance of caution, we notified our field offices and the Joint Terrorism Task Force last week."
Weierman described the reports about al-Qaeda’s possible plans to attack apartment complexes as unsubstantiated and uncorroborated and said they gave no clue where the targeted building were located.
While there are no known recent cases of terrorist bombings of apartment buildings in the United States, they are very well-known in Russia.
A total of about 300 people were killed in a spate of bomb blasts, blamed on Chechen separatists, that rocked multi-apartment housing projects in Moscow and the cities of Buinaksk and Volgodonsk in 1999.
In at least one of these incidents, explosives were moved into a Moscow apartment building in burlap sacks marked "sugar."
Al-Qaeda and the Chechen rebels maintain close ties, according to US officials, with the two groups providing each other military assistance.
The US officials and counterterrorism experts denied the administration of President George W. Bush planned to raise the national security alert level, which currently remains at "yellow," or elevated.
But they warned the suspected new attack could be even more devastating than the Sept. 11 suicide strikes with hijacked airliners slamming against the twin towers of the World Trade Center in New York and the Pentagon building near here.
A fourth airliner crashed in Pennsylvania after passengers apparently thwarted the hijackers.
"There are serious threats that remain," said White House spokeswoman Anne Womack. "We doing our best to collect the information."
She said she could not comment on specific intelligence information from the Central Intelligence Agency about al-Qaeda’s activities.
But intelligence specialists, speaking on condition of anonymity, said there has been an increase in message traffic among al-Qaeda cells in recent weeks.
"That strongly suggests preparations for a new operation," said one of them. "There has been a lot of movement within al-Qaeda recently."
US intelligence analysts believe last month’s attack on a synagogue on the Tunisian resort island of Djerba, which killed 19 people, and a suicide bombing in Karachi, Pakistan, that killed 11 French naval engineers and three Pakistanis, were the work of al-Qaeda.
"That, in conjunction with other factors, indicates al-Qaeda has been able to reorganize itself" since the loss of its bases in Afghanistan, said one of the intelligence experts.
The increased signals traffic among al-Qaeda cells was first reported by The New York Times, which described the intercepted messages as cryptic and ambiguous but with a pattern reminiscent of the period leading up to the Sept. 11 attacks.
The Bush administration has accused al-Qaeda and its leader, Saudi-born Islamist militant Osama bin Laden, of masterminding the Sept. 11 attacks, and launched a massive military campaign to disrupt the group’s activities.