Lawmakers and the Bush administration worried that a new audio message believed to be from Usama bin Laden is a signal that more terror strikes are imminent, even as counterterrorism analysts scoured the recording for clues about Al Qaeda's operations.

Some intelligence analysts have concluded the audiotape almost certainly was made by bin Laden, said a senior law enforcement official, speaking on condition of anonymity.

"He's alive. We have to work on that assumption, and we are," the senior official said Wednesday.

Other U.S. officials were more conservative, saying they believe it probably came from bin Laden. They left open the possibility, however slight, that it is a hoax.

But all said they were treating the tape as a real message from Al Qaeda's missing leader, even as the CIA and National Security Agency conducted a technical analysis of the tape aimed at further authenticating it.

"They can't get to 100 percent certainty, but they're sure," the law enforcement official said. The official, who participated in a high-level briefing by CIA and NSA officials, said analysts were trying to determine whether bin Laden placed cryptic messages in the recording to order followers into action.

President Bush said he was taking the message "very seriously."

"Whoever put the tape out has put the world on notice yet again that we're at war," the president said after a Cabinet meeting at the White House.

The president bristled when asked if bin Laden, blamed for the Sept. 11 attacks, should have been captured sooner by U.S. and coalition forces. "We're making great progress in the war on terror. Slowly, but surely, we are dismantling the terrorist network," he said.

The tape appears to be the first confirmation in a year that bin Laden is alive.

The speaker on the tape sounds undeterred by the loss of bin Laden's home in what was Taliban-ruled Afghanistan or by the death and capture of several of his closest lieutenants.

"Why should fear, killing, destruction, displacement, orphaning and widowing continue to be our lot, while security, stability and happiness be your lot? This is unfair. It is time we get even," he says, sounding as if he is reading from a document.

The message, aired Tuesday on the Arabic television network al-Jazeera, appears aimed at both Westerners and Al Qaeda loyalists, U.S. officials said.

"Assuming it is in fact authentic, it is an effort to boost morale among the rank and file," said one official familiar with the tape, speaking on condition of anonymity. "It is an effort to show members of Al Qaeda that the top leadership is still around. It could also signal future attacks."

There was no change Wednesday in the national threat alert status, which remains at code yellow — the midway point on a scale of five threat levels. This reflects a lack of specific information about impending attacks, officials said.

"We've had increased chatter," said Sen. Richard Shelby, R-Ala., vice chairman of the Senate Intelligence Committee. "I think we'd better be watching for the next hit, probably in a soft place."

Attorney General John Ashcroft said special attention is being paid to "what might be signaled" on the tape, while FBI Director Robert Mueller said the existence of the tape "does and should put us on greater alert."

"There may be individuals in the United States we do not know about who could commit attacks," Mueller said.

The recording appears to have been made during the past two weeks. The speaker appears to refer to the Oct. 28 shooting death of a U.S. diplomat in Amman, Jordan. U.S. officials don't know if Al Qaeda conducted that attack. The speaker also praises the bombing in Bali, Indonesia, last month, that left nearly 200 people dead.

He also takes on issues that resonate in the Islamic world — the U.S. threat of war in Iraq and the ongoing violence between Israelis and Palestinians. He threatens six U.S. allies: Britain, France, Italy, Germany, Canada and Australia.

"We've seen lately that junior members have been able to fan out and effectively engage in terrorist operations not only in the Middle East, but in Southeast Asia and Europe," said Rep. Tim Roemer, D-Ind., a member of the House Intelligence Committee. "If senior leadership is now intact, which this tape would indicate, we've got a bigger and growing problem."

U.S. officials also noted that the tape mentions three top Bush administration officials by name: Vice President Dick Cheney, Defense Secretary Donald H. Rumsfeld and Secretary of State Colin Powell.

"It personalizes things to a degree we hadn't seen before," one official said.

Officials suggested bin Laden would deliver his message by audio, rather than video, because an audiotape gives fewer clues to its origins. It also conceals any potential changes in bin Laden's appearance — either from injuries, illness or efforts at disguise.

Al-Jazeera reporter Ahmad Muaffaq Zaidan said he received the recording in Islamabad, the capital of Pakistan, from an agent of the Al Qaeda leader.

Counterterrorism officials have said that if bin Laden is alive, they believe he is probably in a remote, mountainous area of Pakistan along the country's border with Afghanistan. American officials have never confirmed rumors that bin Laden was wounded or suffering some kind of kidney ailment.