The Arab station said the speaker on the tape was Ayman al-Zawahri, an Egyptian-born surgeon and the closest aide to the al Qaeda terrorist group leader. The U.S. government has offered up to $25 million for information leading to his killing or capture.
It was not immediately possible to verify the authenticity of the recording or determine when it was taped. In Washington, a U.S. official said the CIA was aware of the tape and was looking at it.
The tape emerged one day after a campaign debate between President Bush and Sen. John Kerry, D-Mass., discussed the U.S. war on terror and the search for bin Laden.
The voice sounded like past recordings of al-Zawahri, but it also made an unusual reference to the possibility that al Qaeda's top leaders were not invincible.
"You, youth of Islam, this is our message," he said. "If we die or are detained, continue the path after us, and don't betray God and his prophet, and don't knowingly betray the trust."
The speaker's words came more quickly and energetically, sounding generally more upbeat than previous tapes believed to be from al-Zawahri.
In addition to the United States and Britain, the speaker singled out Australia, France, Poland, Norway, South Korea and Japan, saying their "interests ... are spread everywhere."
"We must not wait more ... or we will be devoured one country after the other," the speaker said. "The youth must not wait for anyone and must begin resisting from now, and take experience and lessons from Iraq and Afghanistan and Chechnya."
He also said the countries cited had given Israel the "means of survival."
The speaker also called for "learned and experience people" to organize what he called "a leadership for the resistance to stand up to the crusader campaign like the holy warriors organized their affairs in Afghanistan, Chechnya or Palestine."
In references apparently to the Russian government, Israel, Yasser Arafat's Palestinian Authority and the U.S.-backed Afghan government, the speaker continued to say the holy warriors had organized "despite the will of the occupier and that of the traitor national governments."
An Al-Jazeera producer said the tape was "supposedly received today," by usual means, which he refused to discuss. The station showed almost four minutes in two clips of a longer recording, and, as usual, was not planning to broadcast the tape in full.
Station officials would not comment on the contents of unaired segments.
The last message purportedly from al-Zawahri surfaced last month just before the third anniversary of the Sept. 11 attacks in the United States. In that videotape, a bearded al-Zawahri proclaimed the United States will be defeated in Iraq and Afghanistan.
The CIA determined with a high degree of confidence that the speaker in that earlier message was al-Zawahri.