In the audio tape broadcast on Aljazeera on Thursday, Osama bin Laden warned that al-Qaida is preparing an attack very soon, but also offers Americans a long-term truce.
Scott McClellan, the White House press secretary, said the United States will not let up in its "war on terror" despite the threats on the tape.
"We do not negotiate with terrorists. We put them out of business," he said.
While warning against downplaying the taped threat, officials at intelligence and law enforcement agencies said there has been no recent increase in "chatter" that can indicate that such an attack is imminent.
The officials discussed the tape on condition of anonymity because intelligence analysis is usually sensitive and because the tape was still being examined.
A Department of Homeland Security official said the agency has no immediate plans to raise the national terror alert, noting that intelligence agencies are still "working to authenticate the tape".
McClellan said the US would act
on 'actionable intelligence'
The national threat level is currently at yellow, the middle of five stages, signifying an elevated risk of attack. It has mostly been at yellow since the 11 September 2001 attacks, but has been raised to orange (high alert) seven times since.
Intelligence officials said authenticating the tape could take between several hours and several days - and perhaps through the weekend.
McClellan said: "If there is any actionable intelligence, we will act on it."
A US counter-terror official said analysts had no reason to doubt an assessment by Aljazeera that the tape was recorded in the Islamic month that corresponds with December.
President Bush was told about the audiotape shortly after an appearance on the economy in Sterling, Virginia, McClellan said.
Intelligence authorities were examining why Bin Laden - if in fact the voice on the tape is verified as his - would be speaking out after more than a year of letting his deputy, Ayman al-Zawahiri, serve as al-Qaida's public face in statements and other communications.
"It isn't clear at this point what it means," said one counter-terror official.
"One would think it might be an effort to demonstrate both to al-Qaida rank-and-file and the public at large that he's still around, given that he hasn't been heard from in such a long time."
Thursday's message to launch new attacks inside the United States, was received with scepticism by some analysts.
"If there is any plan, I don't think he will announce it in advance ... so I would not really attach any value to this apart from the psychological warfare"
Security analyst at the Gulf Research Centre in Dubai
Mustafa Alani, security analyst at the Gulf Research Centre in Dubai, said: "If there is any plan, I don't think he will announce it in advance ... so I would not really attach any value to this apart from the psychological warfare."
"I don't believe they have the capacity now to carry out a major operation, especially in the United States. Their war now is concentrated on two major fronts ... Iraq and Afghanistan."
Alani said it was significant the message had appeared on an audiotape when a video would have had greater impact and could have quelled persistent speculation that Bin Laden is sick or wounded.