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-Zawahiri says Americans and Jewish people are the enemy who "only understand the language of murder".
He also urges Muslims to expel "criminals" from the Middle East and strike at the embassies and commercial interests of the United States, Britain, Australia and Norway.
Osama Bin Laden's al-Qaeda network is believed to have been behind recent suicide attacks in Saudi Arabia and Morocco, 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.
The move follows the decision to put the US as a whole on orange alert - the second-highest statge of readiness.
And in another development, Saudi security sources have said that three al-Qaeda suspects arrested in the western port of Jeddah were trying to hijack a passenger plane and use it in a suicide attack.
The statement broadcast on al-Jazeera calls for the use of violence instead of demonstrations and talks.
"The crusaders and the Jews do not understand but the language of killing and blood," it says.
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."
BBC Middle East analyst Roger Hardy says the voice on the tape sounds like Mr Zawahiri's.
It is unclear when the message was recorded - although a reference to the war in Iraq suggests it was made during that conflict.
The speaker condemns Arab countries he says helped the US-led campaign, naming Saudi Arabia, Kuwait, Qatar, Bahrain, Yemen and Jordan.