In a letter sent to the leader of Al Qaeda in Iraq in the past few weeks, Zawahiri claims that it is defeating U.S. forces and urges followers to expand their campaign of terror.
He conjures a vision of an Islamic state comprising Lebanon, Palestine and Syria, where Al Qaeda has already gained its first footholds.
The goal of an Islamic "greater Syria," first outlined by Zawahiri two years ago, is detailed in the letter amid growing concern about the activities of new groups under Al Qaeda's influence in the countries concerned.
In Lebanon yesterday, Islamist militants of various nationalities from Fatah al-Islam were holding out under renewed army bombardment in Nahr al-Bared camp near Tripoli. Many fought and trained in Iraq.
The little-known Jaish al-Islam claims to be holding Alan John-ston, the BBC journalist, in Gaza, where militants disillusioned with Hamas are said to be drifting to Al Qaeda.
Jaish al-Islam has demanded the release of a woman who tried to blow herself up during the 2005 bombing of a wedding reception in the Jordanian capital of Amman, an operation ordered by Al Qaeda in Iraq.
In Syria, the Al Qaeda leader Abu Musab al-Suri is believed to have been arrested, but cells are said to be recruiting in Palestinian camps near Damascus, the capital. Al Qaeda Web sites are demanding the overthrow of President Bashar al-Assad.
Zawahiri's letter was sent to Abu Hamza al-Muhajer, the leader of Al Qaeda in Iraq, and intercepted by a Middle Eastern intelligence service.
A senior American security source said he was aware of the letter and Al Qaeda's growing emphasis on spreading jihad through a volatile region.
The source said Zawahiri, a Sunni, was determined to prevent Lebanon falling into the hands of the Shiite Hezbollah movement, which has tried to bring down the government.
"Al Qaeda is trying very hard to seize a foothold in Syria," the American source added.