(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."
It cited bin Laden's "martyrdom" and prayed "to the Almighty to raise him to his heavens and reward him for his good deeds for us and the Islamic nation."
Al-Zawahiri's official appointment to the terrorist group's top post -- just a few days before his 60th birthday -- is "not surprising," the U.S. counterterrorism official said. "He had been the number two. It was surprising it took a couple of days for al Qaeda to acknowledge bin Laden's death and this long to say Zawahiri is the new leader."
One of the FBI's most wanted terrorists, al-Zawahiri has played a defining role in al Qaeda. He was indicted for his alleged role in the August 7, 1998, bombings of the United States Embassies in Dar es Salaam, Tanzania, and Nairobi, Kenya, that killed 224 civilians and wounded more than 5,000 others, according to the National Counterterrorism Center, part of the U.S. federal government.
The FBI is offering a $25 million reward for information leading to his capture, the same amount as the reward for bin Laden.
Al-Zawahiri's wife and three children were killed in December 2001 in a U.S. attack on the family's residence in Afghanistan.
Asked whether the United States is any closer to finding al-Zawahiri as a result of information obtained at bin Laden's compound or from leads resulting from a recent al-Zawahiri videotaped message about bin Laden's death, the U.S. counterterrorism official said only, "Obviously, he has been and will continue to be of high interest."
In a nearly half-hour video released this month, al-Zawahiri eulogized bin Laden, saying he "went to his God martyred because he said no to America."
Al-Zawahiri also criticized the "treacherous" Pakistani government and warned that America is "not facing individuals or groups but an entire (Islamic nation that) rose from the ashes in a jihadi awakening to face its oppressors."
There will probably not be changes in U.S. counterterrorism strategy now that al-Zawahiri is leading al Qaeda, the U.S. counterterrorism official said.
The al Qaeda statement cannot be authenticated by CNN, but it appeared on radical Islamist sites known for posting similar statements and recruitment videos by other al Qaeda figures.
It said the group will not shift its policies and pledged its support to, among others, the Taliban and leader Mullah Omar as the group battles U.S. and NATO presence in Afghanistan.
The posting makes no mention of the pro-democracy uprisings in much of the Arab world that have forced the exit of some longtime leaders.
Paul Cruickshank, a CNN terrorism analyst, said al-Zawahiri may have a hard time following bin Laden.
"Zawahiri has none of the charisma of bin Laden," Cruickshank said. "He wants to inspire people, not just who are joining the al Qaeda organization, but people who have never joined the al Qaeda organization and are trying to launch attacks in their name. Without bin Laden there anymore, they won't be as inspired."
Former CIA Officer Phil Mudd added al-Zawahiri is "very poorly respected."
"He is seen as a difficult man to work with," Mudd said. "He has no sense among the work force in al Qaeda, the kind of prestige that bin Laden had."
The U.S. counterterrorism official said the United States does not know whether al Qaeda members have pledged their loyalty to al-Zawahiri.
Born into a wealthy family in Cairo, al-Zawahiri is a physician and founding member of the Egyptian Islamic Jihad, a militant organization that opposed the secular Egyptian government of Hosni Mubarak and sought its overthrow through violent means.
Like bin Laden, al-Zawahiri also went to Afghanistan during its fight against the Soviets, although he was there primarily to offer medical expertise.
By the 1990s, he refocused his attention on undermining and attacking the Egyptian government and, eventually, the United States.
In 1998, when the Egyptian Islamic Jihad effectively merged with al Qaeda, he sent a fax to the Al-Hayat newspaper warning Americans. Three days later, August 7, suicide truck bombers carried out the embassy attacks.
The al Qaeda statement Thursday showed the group's commitment to leading jihad efforts around the globe.
Citing Palestinian territories, Somalia and Chechnya, the statement said al Qaeda "will remain on the path of righteousness as a coherent, solid, cemented rank, with one word that will bind us together, with hearts of affinity, and under one wholesome banner, battling one enemy. Even if the names may vary, there is no failing, no hesitation, no surrender."
CNN's Saad Abedine, Hamdi Alkhshali and Carol Cratty contributed to this report.