Bin Laden and Zawahiri are comrades in arms
The Arabic television channel al-Jazeera has broadcast what it says is an audio tape of al-Qaeda's second-in-command urging Muslims to carry out more suicide attacks.
In the tape, a man identified by the channel as Ayman al-Zawahri urges attacks on the embassies and interests of the United States, Britain, Australia and Norway, to drive them from Muslim countries.
He warns that the US and its allies want "dismembered semi-states" around Israel, and condemns Arab countries that supported the US-led war on Iraq.
Addressing the people of Iraq, he predicts news to "heal your breasts" in the coming days - although it is not clear when the message was recorded, and a reference to the war in Iraq suggests it was made during the conflict.
Osama Bin Laden's al-Qaeda network is believed to have been behind last week's suicide attacks in Saudi Arabia, in which dozens of people were killed.
The tape was broadcast as the American military put its bases in the US on the highest possible alert amid fears of an imminent terror attack.
In another development, Saudi Interior Minister Prince Nayef on Wednesday denied reports that three Moroccans arrested this week in the western port of Jeddah were trying to hijack a passenger plane to use it in a suicide attack.
The statement broadcast on al-Jazeera calls for the use of violence instead of protests and peace talks.
"Demonstrations will not... protect your jeopardised holy places or expel an occupying enemy," the statement said.
"The crusaders and the Jews do not understand but the language of killing and blood."
We will hunt the terrorists in every dark corner of the earth
And in a reference to the 11 September 2001 attacks on the US, it adds: "Learn from your 19 brothers who attacked America in its planes in New York and Washington."
The speaker condemns Arab countries he says helped the US-led campaign, naming Saudi Arabia, Kuwait, Qatar, Bahrain, Yemen and Jordan.
BBC Middle East analyst Roger Hardy says the voice on the tape sounds like Ayman al-Zawahri's.
He adds that al-Qaeda has an obvious interest in releasing such a tape now - to show it is still active, and to stoke the tension created by last week's suicide bombings in Saudi Arabia and Morocco.
The tape comes after the American authorities put the US on orange alert - the second-highest state of readiness - and its military facilities on full alert.
US President George W Bush said on Wednesday that America would not relent in its war on global terror.
Al-Qaeda has been blamed for the Saudi attacks
"We will hunt the terrorists in every dark corner of the earth," he told the US Coast Guard in Connecticut.
"We will not permit terror networks or terror states to threaten the world with weapons of mass destruction - as we have shown in the battle with Iraq," he added.
Norway, meanwhile, says it has no idea why it was singled out for attacks on its interests.
BBC's Lars Bevanger in Oslo says people there are baffled because Norway has a reputation for its attempts to bring peace to the Middle East.
Norway has some troops in Afghanistan, but analysts say it is small and other countries have larger numbers.
Our correspondent says the Norwegians are hoping al-Qaeda simply got its geography wrong but they are nonetheless taking the threat seriously.