The tape, aired by Aljazeera, was the first public communication from Bin Laden since December 2004.
An agency official, speaking on condition of anonymity because the official was not authorized to speak by name, said: "Following technical analysis of the Osama bin Laden tape aired today, the CIA assesses that it was the voice of Osama bin Laden."
The official provided no details about how the Central Intelligence Agency concluded that the voice was that of Bin Laden.
The agency's determination was sure to intensify the scrutiny that officials from the United States and other countries will give to the recording.
In the tape, bin Laden said he was directing his message to the American people after polls showed that "an overwhelming majority of you want the withdrawal of American troops from Iraq but (Bush) opposed that desire".
The last audiotape from bin Laden was broadcast in December 2004. In that recording, he endorsed Abu-Musab al-Zarqawi as his deputy in Iraq and called for a boycott of Iraqi elections.
"That's the message of a powerful organisation, not a weakened one"
analyst, Jane's Defense Weekly
Jeremy Bennie, a terrorism analyst for Jane's Defense Weekly, said Bin Laden appeared to be "playing the peacemaker, the more statesmanlike character" with his offer of a truce.
"They want to promote the image that they can launch attacks if and when it suits them. That's the message of a powerful organisation, not a weakened one. They want us to believe they are in control," he said.
The mention of rebuilding Iraq and Afghanistan may be a recognition of divisions among the ranks of Islamist fighters over the battle in Iraq by Bin Laden's ally, al-Zarqawi, who has come under criticism by some radicals for attacks on Iraqi civilians.
Richard Clarke, a former White House anti-terrorism chief, said: "The initial significance of this is that he is still alive.
"The only new element in his statement is that they are planning an attack soon on the United States.
"Would he say that and risk being proved wrong, if he cannot pull it off in a month or so?" Clarke asked.
Of the truce offer, Clarke said: "I think it is 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 is too reviled."
Intelligence authorities were examining why Bin Laden would be speaking out after more than a year of letting al-Zawahiri serve as al-Qaida's public face in statements and other communications.
A tape showing al-Zawahiri was
aired just weeks ago
Bin Laden issued numerous tapes in 2003 and 2004, calling for Muslims to attack US interests and threatening attacks against the United States.
In a 15 April 2004 audiotape, he vowed revenge against the US for Israel's assassination of Hamas founder Shaikh Ahmed Yassin - and at the same time offered a truce to European countries.
Bin Laden appeared in a video released in October 2004, just ahead of US presidential elections, saying the United States can avoid another September 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-Qaida had previously offered them.