Al Qaeda released a string of videos for the anniversary, showing increasingly sophisticated techniques as it tried to demonstrate that it remains a powerful, confident force five years into the United States' war on terror.
One video showed images of the planes striking the World Trade Center, lionizing the 19 suicide hijackers as men "who changed history." Another was a 91-minute documentary-style video in which Osama bin Laden is seen smiling and chatting with the planners of the Sept. 11, 2001, attacks in an Afghan mountain camp.
Al-Zawahri spoke in the third, longest video, warning Americans that more attacks were to come.
"We have repeatedly warned you and offered a truce with you. Now we have all the legal and rational justification to continue to fight you until your power is destroyed or you give in and surrender," he said. "The days are pregnant and giving birth to new events."
His comments pointed to new fronts for al Qaeda attacks. The terror network has had few operations in Lebanon, Israel or in the Gulf region — except for in Saudi Arabia, where its branch carried out a campaign of violence in recent years but has been heavily damaged by a government crackdown.
But both Lebanon and Israel have warned of a possible growing al Qaeda presence.
"We have seen over the last months increased Al Qaeda activity in our area," in Lebanon, Jordan and Egypt's Sinai peninsula, Israeli Foreign Ministry spokesman Mark Regev said.
"We've seen an attempt by al Qaeda to also infiltrate in Gaza and even in the West Bank, so we take the threat very seriously and we're taking the appropriate countermeasures," he said, without elaborating.
Addressing the United States, al-Zawahri said "you should not waste your time" reinforcing troops in Iraq and Afghanistan, "because they are doomed to defeat."
"Instead, you have to reinforce your troops in two regions. First is the Gulf, where you will be thrown out after you are defeated in Iraq, at which point your economic ruin will be achieved," he said. "The second is Israel, because the jihad reinforcements are getting closer to it."