US President George W Bush has brushed aside demands from Osama Bin Laden's deputy that the US leave Iraq, saying the US would "complete the job" there.
Ayman al-Zawahri warned that there would be further violence unless the US and its allies withdrew from Iraq.
His comments were made in a videotape broadcast on the Arab satellite television network al-Jazeera.
Mr Bush dismissed Zawahri's ideology as "dark, dim and backwards" and said Iraqis wanted to live in freedom.
'Clash of ideologies'
"He's threatening. They have come up against a nation that will defend itself," he said.
"The Iraqis want to live in a free society. Zawahri doesn't want them to live in a free society. And that's the clash of ideologies: freedom versus tyranny," he added.
In the tape, Zawahri - dressed in a white tunic and black turban and posed next to a rifle - warned other nations to leave Muslim lands to avoid further violence.
He threatened an escalation in attacks, saying the losses in Afghanistan and Iraq were only those of "initial clashes".
"If you continue the same policy of aggression against Muslims, God willing, you will see the horror that will make you forget what you had seen in Vietnam," he said.
The al-Qaeda deputy also said that the foreign policy decisions of Prime Minister Tony Blair were directly responsible for the London attacks.
Excuse to attack
Mr Blair denies his policies provoked the 7 July bombs, which killed 56. His office has refused to comment on the latest al-Qaeda tape.
Mr Blair has said the Iraq war is merely an excuse for those who want to attack the UK.
In a scathing attack, US Defence Secretary Donald Rumsfeld also criticised those who, he said, were clinging to a discredited theory which viewed the London attacks as retaliation for war in Iraq.
He labelled the theory "nonsense".
Mr Blair has acknowledged Iraq is being used to recruit terrorists, but insisted the roots of extremism were much deeper.
Zawahri last appeared in a video in June, saying Muslims should not rely on peaceful protests but should also use violence. He also appeared in a video in February.
The Egyptian-born Zawahri is thought to be Bin Laden's deputy and to have been hiding in the rugged border areas of either Pakistan or Afghanistan.