(CNN) -- Ayman al-Zawahiri, the longtime deputy to Osama bin Laden, will take over leadership of al Qaeda, according to a statement posted on several jihadist websites Thursday. The United States believes that al-Zawahiri has indeed taken the helm of the terrorist network, according to a U.S. counterterrorism official. Al-Zawahiri was widely regarded as al Qaeda's de facto leader since U.S. special forces killed Osama bin Laden at a hideout in Abbottabad, Pakistan, on May 2. "Hereby the General Command of the Qaeda al-Jihad -- and after the end of the consultations -- we declare that Sheikh Dr. Abu Muhammad Ayman al-Zawahiri (may God bless him) will take over the responsibility of command of the group," the statement said, attributed to al Qaeda's "general command."
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NEW: U.S. believes al-Zawahiri is leading al Qaeda, a U.S. counterterrorism official says
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NEW: It's surprising it took this long, the U.S. official says
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NEW: Zawahiri's video message this month contained a warning to America
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Statement pledges support to Mullah Omar but does not mention Arab uprisings
"He has no sense among the work force in @placeholder, the kind of prestige that bin Laden had."