Al-Zawahiri addressed the US and its citizens in a video, seen by Aljazeera on Monday, which appeared to be new.
He said, "You gave us every legitimacy and every opportunity to continue fighting you ... You should worry about your presence in the [Persian] Gulf, and the second place you should worry about is Israel.
"Your leaders are hiding from you the true extent of the disaster, and the days are pregnant and giving birth to new events with Allah's permission and guidance."
He also said al-Qaeda was justified in continuing its fight against America as the US administration had "ignored our offer of a truce", adding US troops fighting in Iraq and Afghanistan "were doomed".
Al-Zawahiri refers to Israel's bombardment of Lebanon this summer, the capture of Israeli soldiers by Hezbollah and fighters in Gaza in the video, entitled Hot Issues, leading experts to believe it is probably new.
He says that Israel was attacking Lebanon and Gaza only because Egypt had left the "field of conflict with Israel".
As-Sahab website, al-Qaeda's media arm, posted on Sunday the full video of a smiling Osama bin Laden and other commanders in a mountain camp, apparently planning the September 11 attacks, excerpts of which were first aired by Aljazeera last Thursday.
The documentary-like retrospective of the five years since the attacks was unusually long - 91 minutes, split into two segments and sophisticated in its production quality compared to previous al-Qaeda videos.
It included the last testament of two of the September 11 hijackers, Wail al-Shihri and Hamza al-Ghamdi, and showed bin Laden strolling in the camp, greeting followers.
The video had slicker production
than previous al-Qaeda tapes
"Among the devout group which responded to the order of Allah and the order of his messenger were the heroes of September 11, who wrote with their blood the greatest pages of modern history," the narrator said, referring to the hijackers who flew planes into the Pentagon and the World Trade Centre.
Al-Shihri and al-Ghamdi were each shown speaking to the camera, their image superimposed over background pictures of the crumbling World Trade Centre towers and the burning Pentagon, as well as a model of a passenger jet.