Al-Zawahiri was bin Laden's longtime deputy and turns 60 on Sunday. He's believed to be operating from somewhere near the Pakistan-Afghanistan border.
The Egyptian-born surgeon is a fiery ideologue who is known for his deep hatred of the West and helped plan the 9/11 attacks. He has been credited with bringing tactical and organizational cunning to al Qaeda, which has found itself increasingly decentralized and prone to internal disputes following its expulsion from Afghanistan after the 2001 U.S. invasion.
The move comes at a time the terror network is struggling for relevance as a wave of Arab uprisings has threatened to leave it marginalized.
Al-Zawahiri pledged earlier this month to avenge the U.S. raid on May 2 that killed bin Laden.
The Associated Press contributed to this report.