Al Qaeda is preparing for attacks in the United States, but the terror group is offering a conditional truce to help rebuild Iraq and Afghanistan, Usama bin Laden said in excerpts of an audiotape aired Thursday on Al-Jazeera.

The CIA later said the tape is authentic, but the FBI was not so sure.

"Following technical analysis of the Usama bin Laden tape aired today, the CIA assesses that it was the voice of Usama bin Laden," said the agency official, who provided no details about how that conclusion was made.

FBI spokesman John Miller, however, said it's not 100 percent certain bin Laden is even alive. The United States has placed a $25 million bounty on the terror leader's head.

"The tape's being analyzed right now — not just by the FBI but our partners in the intel community — one, to gauge its authenticity, though to date there hasn't been a false bin Laden tape that's come out, and second, to analyze its content and message," he said.

If authentic, it would be the first communication from the terror mastermind in more than a year.

Vice President Dick Cheney said the tape showed that Al Qaeda has been hobbled, because "they didn't have the ability to do anything on video" and because it had been so long since bin Laden had been heard from. However, he said the threat from Al Qaeda continues, noting "ample evidence of continued plotting against the United States."

"I think we have to assume that the threat is going to continue for a considerable period of time," the vice president told FOX News in an interview Thursday. "Even if bin Laden were no longer to be a factor, I still think we'd have problems with Al Qaeda."

No matter what, White House officials said the United States will remain firm in its fight against terror.

"Clearly the Al Qaeda leaders and other terrorists are on the run and under a lot of pressure," White House spokesman Scott McClellan said. "We do not negotiate with terrorists; we put them out of business."

U.S. counterterror officials said Thursday they have seen no specific or credible intelligence or noticed any increase in "chatter" that would indicate an upcoming Al Qaeda attack on the country and there are no current plans to raise the country's terror alert level.

Unless there's an overt threat — there's not much more the FBI can do for security, one FBI spokesman said, since "how much more can you ratchet things up?"

Though the Homeland Security Department said it would not raise the national threat alert at this time, the tape prompted increased security at Los Angeles International Airport and other precautions at the city's port, water and power facilities.

"At this time, we lack corroborating information suggesting that Al Qaeda is prepared to attack the United States in the near term," said Homeland Security spokeswoman Michelle Petrovich. "But we recognize that Al Qaeda remains committed to striking the homeland."

Homeland Security officials alerted states about bin Laden's comments in a routine call Thursday morning, Petrovich said.

In Los Angeles, Mayor Antonio Villaraigosa said city police deployed additional resources at their airport and "posted signage indicating that bomb sniffing dogs and searches will occur frequently." He described the measures as precautionary, as there were no known threats to the city. The Port of Los Angeles and the Department of Water and Power also planned to take precautionary measures, according to Villaraigosa's statement.

Sharon Gang, a spokeswoman for District of Columbia Mayor Anthony A. Williams, said the capital was not raising its terror alert level. Across the Potomac River in Virginia, the information did not trigger any alarms, said Steve Mondul, the state's deputy preparedness director.

"The basic header we put on it was, everyone should be a little more alert, and watch this space for further instructions," said Mondul, who participated in the Homeland Security call and then passed on the information to Virginia law enforcement, military, health and transportation agencies.

The voice on the tape said heightened security measures in the United States are not the reason there have been no attacks there since Sept. 11, 2001, but "because there are operations that need preparations, and you will see them."

"Our people are able to infiltrate through your security measures no matter how strong," he continued.

He said insurgents are winning the conflict in Iraq and warned that security measures in the West and the United States could not prevent attacks there.

"The proof of that is the explosions you have seen in the capitals of European nations," he said.

"The delay in similar operations happening in America has not been because of failure to break through your security measures. The operations are under preparation and you will see them in your homes the minute they are through [with preparations], with God's permission."

Speaking before The Manhattan Institute for Policy Research in New York on Thursday, Cheney said: "Our nation has been protected and it's been more than luck."

The voice on the tape also proposed a "long-term truce with fair conditions" to the U.S. military.

"We are a nation that God has forbidden to lie and cheat. So both sides can enjoy security and stability under this truce so we can build Iraq and Afghanistan, which have been destroyed in this war," it said. "There is no shame in this solution, which prevents the wasting of billions of dollars that have gone to those with influence and merchants of war in America."

U.S. intelligence officials believe the tape was made in December. One counterintelligence official said that based on this tape alone, there are no plans at this time to raise the terrorist threat level for the United States.

The national terror threat level currently stands at from yellow, the middle of five grades, signifying an elevated risk of attack. The government has raised the alert level to orange, signaling a high threat risk, seven times since the attacks of Sept. 11, 2001.

Voice: Bush Should Listen to Polls

In an Arabic transcription of the entire tape on the Al-Jazeera Web site — but not aired — bin Laden makes an oblique reference to how to prevent new attacks on the United States, but does not specify if these are conditions for a truce.

"If I were president, I would stop the attacks on the United States: First I would apologize to all the widows and orphans and those who were tortured. Then I would announce that American interference in the nations of the world has ended," he said.

The speaker refers to an alleged comment by President Bush about bombing the Qatar headquarters of Al-Jazeera, which was first reported in the British press on Nov. 22. He also refers indirectly to the July 7 bombings in London that killed 56 people.

The speaker said Bush should have paid attention to polls that showed most Americans want U.S. troops withdrawn from Iraq instead of saying that it's better to fight the terrorists on their land rather than on American soil.

"My answer to this is that the war in Iraq is raging with no calm, and operations in Afghanistan are on the rise and the Pentagon numbers indicate that your casualties are increasing in addition to the enormous material losses," the voice said.

Al-Jazeera's editor-in-chief, Ahmed al-Sheik, would not comment on when or where the tape was received. He said the full tape was 10 minutes long. He said the tape seemed to have been made "recently."

FOX News contacts around Pakistan and Afghanistan say they were fully expecting some kind of communication from a top Al Qaeda figure following the alleged U.S. attack last Friday on a Pakistan compound where it was believed some high-profile Al Qaeda operatives were invited to dinner. But it was believed that the communication may come from bin Laden's top lieutenant, Ayman al-Zawahiri.

There was no mention of the attack on the segments that were broadcast.

"I think he's trying to get people activated around the world," former CIA officer Ron Marks told FOX News. "The [Al Qaeda] leadership on the field is pretty much fighting on its own in this time, especially in Iraq, where, while they embrace him, they're pretty much running their own operations."

"They want to promote the image that they can launch attacks if and when it suits them. That's the message of a powerful organization, not a weakened one. They want us to believe they are in control," added Jeremy Bennie, a terrorism analyst for Jane's Defense Weekly.

Joint Chiefs of Staff Chairman Peter Pace told FOX News in an interview Thursday that even if bin Laden is alive, his terror network has been weakened and is on the run, and many of his top deputies have been killed.

Over the past year, there has been much speculation about bin Laden's whereabouts and even whether he was still alive.

The tape apparently provides no definitive answers to either question, but there was speculation that it might be an attempt to show supporters that bin Laden was still around.

"He has made threats before, but there hasn't been a public utterance for a long time, and for that reason no one is being dismissive of it," said one counterterror official, speaking on condition of anonymity while the tape was still being analyzed.

President Bush was told about the audiotape Thursday, White House spokesman Scott McClellan said.

McClellan on Thursday repeated Bush's stance that the United States will stay in Afghanistan and Iraq, since that's the best way to defeat terrorists where they thrive and to win the "ideological struggle" against extremism.

"It's important we do not let up and we do not stop until the job is done," McClellan said.

As for the whereabouts of bin Laden, the spokesman added: "I think clearly he's on the run, he's under a lot of pressure just like a lot of Al Qaeda leaders are. That's why we're going to continue pursing them and continue to bring them to justice … this is a war we will not let up until we prevail."

There has been no audiotape from bin Laden broadcast since December 2004. In that recording, he endorsed Abu-Musab al-Zarqawi as his deputy in Iraq and called for a boycott of Iraqi elections. Al-Jazeera said the latest tape was recorded in the Islamic month that corresponds with December.

Since the Sept. 11 attacks, the longest bin Laden had gone without issuing a public statement — audio or video — was just over nine months.

Bin Laden issued numerous tapes in 2003 and 2004, calling for Muslims to attack U.S. interests and threatening attacks against the United States.

In an April 15, 2004, audiotape, he vowed revenge against the United States for Israel's assassination of Hamas founder Sheik Ahmed Yassin — and at the same time offered a truce to European countries.

Bin Laden appeared in a video released October 2004, just ahead of U.S. presidential elections, saying the United States can avoid another Sept. 11 attack if it stops threatening the security of Muslims.

Since December 2004, al-Zawahiri has issued a number of video and audiotapes, including one claiming responsibility for the London attacks, which he said came after Europe rejected the terms of a truce Al Qaeda had previously offered them.

The mention of rebuilding Iraq and Afghanistan may be a recognition of divisions among the ranks of Islamic militants over the insurgency in Iraq by bin Laden's ally, al-Zarqawi, who has come under criticism by some radicals for attacks on Iraqi civilians.

Former White House antiterrorism chief Richard Clarke said bin Laden's threats shouldn't be taken lightly.

"The only new element in his statement is that they are planning an attack soon on the United States," Clarke said. "Would he say that and risk being proved wrong, if he can't pull it off in a month or so?"

Of the truce offer, which Clarke said bin Laden has made before, "I think it's designed to make him look more reasonable in Arab and Muslim eyes. He's a very sophisticated reader of world opinion and American opinion, and he obviously knows he can't affect American thinking. He's too reviled."

FOX News' Bret Bair, Mike Levine, Greg Palkot and The Associated Press contributed to this report.