The Federal Bureau of Investigation has warned that the al-Qaeda network could be planning "spectacular" attacks on the United States - targeting in particular national landmarks or major industries.

Al-Qaeda may favour spectacular attacks that meet several criteria: high symbolic value, mass casualties, severe damage to the US economy and maximum psychological trauma

FBI memo

Despite the warning, the country's level of alert has not been raised.

BBC Washington correspondent Justin Webb says the White House reaction to the FBI warning has been lukewarm.

Americans would be much more concerned if the FBI had been backed up by dire warnings from the White House and some change had been made to the colour coded level of alert.

National security adviser Condoleezza Rice said there was no new information in the latest warning. It was, she said, "a summary of intelligence as we know it".

Our correspondent says that leaves most Americans worried but not knowing exactly how worried they should be or what they should be worried about.

The FBI memo has created a stir because of the strong language it employs.

It comes just days after the release of an audio tape reportedly made by Osama Bin Laden, in which the al-Qaeda leader apparently praised recent attacks such as last month's bombing in Bali.

'Softer targets'

"In selecting its next targets, sources suggest al-Qaeda may favour spectacular attacks that meet several criteria: high symbolic value, mass casualties, severe damage to the US economy and maximum psychological trauma," the FBI warning says.

Al-Qaeda's next attack may rely on conventional explosives and low-technology platforms

FBI memo

"The highest priority targets remain within the aviation, petroleum, and nuclear sectors as well as significant national landmarks."

The memo suggests that small-scale operations against "softer targets" would be easier for sleeper cells of the network already established within the US to carry out.

"Al-Qaeda's next attack may rely on conventional explosives and low-technology platforms such as truck bombs, commercial or private aircraft, small watercraft or explosives easily concealed and planted by terrorist operatives," it says.

Additional precautions

White House spokesman Scott McClellan said the information was not specific about timing, location or the type of attack.

"While we are not raising the threat level, we are taking additional precautions to meet the threat. We take these threats seriously," he said.

The US is currently on yellow alert - in the middle-level of a scale established following the 11 September attacks, which were blamed on al-Qaeda.