The alleged voice of Zawahri urges strikes against the US

US Secretary of State Colin Powell termed the broadcast unfortunate and said that giving air time to terrorists just heightened tensions.

Al-Jazeera, which is based in Qatar, said the audio tape was the voice of Ayman al-Zawahri, a top aide to Osama bin Laden.

The tape urged Muslims to strike at the embassies and commercial interests of the US, Britain, Australia and Norway.

All it does is heighten tension throughout the region to allow this kind of terrorist to have access to the airwaves Colin Powell

It was broadcast as the American military put its bases in the US on the highest possible alert amid fears of an imminent attack.

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 American embassy in the Norwegian capital, Oslo, has been closed in the wake of the audio tape's threats.

Tape 'stokes tension'

Mr Powell said the tape "spreads more hatred throughout the world".

"We think it was unfortunate that al-Jazeera ran the tape. All it does is heighten tension throughout the region to allow this kind of terrorist to have access to the airwaves," he added.

The crusaders and the Jews do not understand but the language of killing and blood Voice purported to belong to Ayman al-Zawahri Full statement

Mr Powell said he did not know whether the tape was authentic or accurate.

On it, a voice purporting to be that of Ayman al-Zawahri 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.

It is not clear when the message was recorded, but a reference to the war in Iraq suggests it was made during the conflict.

Referring 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.

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.