Bush: Warned of "general threats"

The US intelligence community has already been heavily criticised for its failure to detect warning signs of the 11 September suicide attacks on New York and Washington.

The attacks, which saw hijacked aircraft flown into the World Trade Center and the Pentagon, are believed to have been carried out by Bin Laden's al-Qaeda terror group.

However, Mr Fleischer said the information received by the president dealt with conventional hijackings - not the use of planes as missiles to attack buildings.

"We had general threats involving Osama Bin Laden around the world and including in the United States," he said.

Former CIA field officer Robert Baer told the BBC that alerts of this severity were received by the White House all the time.

"These unfocused threats are always sent up through the system and the system is so risk averse that rather than trying to make the judgment at a lower level, they send everything to the White House"

But BBC Washington correspondent Justin Webb says the timing of this admission is significant, as a congressional committee is about to start hearings into intelligence failings before 11 September.

The White House did not want to be put on the defensive with leaks about what the president knew, our correspondent says.

Flight school warning

Meanwhile, the White House has denied that a memo last July from the Federal Bureau of Investigation's Arizona office could have prevented the attacks had it been acted on.

The memo is reported to have warned that groups like al-Qaeda might have sent students to flight schools in the US, but none of the people identified in the document had any connection with the attacks.

Zacharias Moussaoui: Awaiting trial

Even though the memo was reviewed, the FBI did not take any action on its central recommendation - that flight school records and visa applications by foreign students be cross-referenced.

And the agency did not connect the memo with the case of Zacharias Moussaoui, a Frenchman of Moroccan descent who was arrested in August after seeking flight training in Minnesota - and saying he was not particularly interested in learning to land aeroplanes.

Mr Moussaoui is facing trial in the US, accused of conspiring with Bin Laden, the hijackers and others to commit the 11 September attacks.

The FBI Director, Robert Mueller, has repeatedly said he wished agents had acted more aggressively in putting the Arizona and Minnesota leads together.

Following the attacks, the FBI is to create a special counter-terrorism unit to oversee all its terrorism investigations.

A Washington-based "super-squad" will be made up of hundreds of agents and analysts, as well as an office of intelligence, headed by a former employee of the Central Intelligence Agency.