In a bid to play down US claims of achieving progress in the "war on terror", al-Zawahiri said al-Qaida was expanding, spreading and getting stronger, and called on supporters to attack oil sites in Muslim countries.
"All the lies that [US President George] Bush tries to delude the Americans with, saying that he destroyed half, or three quarters of al-Qaida are but nonsense merely in his own head," al-Zawahiri said in the video.
"We want to tell all the Muslims and the mujahidin al-Qaida, thank God, is expanding and increasing in strength," he said.
"I call on mujahidin to focus their attacks on Muslims' embezzled oil. Most of its revenues go to the enemies of Islam; meanwhile most of what is left is taken by the thieves who are ruling our countries."
Al-Zawahiri accused the US of trying to control Iraq's wealth and maintain military bases away from the reach of mujahidin even after it eventually withdraws from the country.
He claimed responsibility for London's July attacks saying that the British policy in Iraq and Palestine, and its hostility to Islam, justified what happened in London.
He also called on "resistance factions" in Iraq to unite, and urged Arabs, Kurds and Turkmen to form one political entity to liberate Iraq.
It was al-Zawahiri's first appearance since a videotape on 19 September when he claimed responsibility for the deadly attacks on London's transport system in July.
Aljazeera on 4 August aired a video of al-Zawahiri warning Britain and the US of more attacks, exactly four weeks after the London bombing, which was followed by failed copycat attacks a fortnight later.
The al-Qaida network has claimed attacks around the world, including the 11 September 2001 attacks on the US.
The US believes al-Zawahiri, who has a $25 million bounty on his head, is the main strategist and key ideologist in the chain of command of al-Qaida.
Al-Zawahiri has been implicated in the 1981 assassination of the then Egyptian president Anwar al-Sadat, and the massacre of foreign tourists in the Nile resort of Luxor in 1997.
An eye surgeon by training and from a wealthy Egyptian family, he faces a death sentence in Egypt.
Before becoming Bin Ladin's right-hand man, he was the leader of the Jihad group, which spearheaded together with the Jamaa Islamiyah organisation a wave of attacks that rocked Egypt in the 1990s.