Al-Qaida's second-in-command Ayman al-Zawahri speaks from an unknown location, in this still image taken from video uploaded on a social media website on June 8, 2011. Osama bin Laden's long-time lieutenant Ayman al-Zawahri has taken command of al-Qaida after the killing of the group's founder and leader, an Islamist website said on June 16, 2011. [Photo/Agencies]

CAIRO - Osama bin Laden's longtime second-in-command, Ayman al-Zawahri, has taken control of al-Qaida, the group declared Thursday, marking the ascendancy of a man driven by hatred of the United States who helped plan the 9/11 attacks.

Al-Zawahri is considered the organizational brain of the terror group, highly skilled at planning and logistics. Analysts said he could set his sights on a spectacular attack and on building up al-Qaida's already robust presence in Yemen to establish his leadership credentials.

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His fanaticism and the depth of his hatred for the United States and Israel are likely to define al-Qaida's actions under al-Zawahri's tutelage. In a 2001 treatise that offered a glimpse of his violent thoughts, al-Zawahri set down al-Qaida's strategy: to inflict "as many casualties as possible" on the Americans.

"Pursuing the Americans and Jews is not an impossible task," he wrote. "Killing them is not impossible, whether by a bullet, a knife stab, a bomb or a strike with an iron bar."

Al-Zawahri's hatred of America was also deeply personal: His wife and at least two of their six children were killed in a US airstrike following the 2001 US invasion of Afghanistan after the 9-11 attacks.

The Egyptian-born al-Zawahri had been expected to inherit al-Qaida's leadership, although the delay in announcing his succession led some counterterrorism analysts to speculate about a power struggle following the May 2 killing of bin Laden in a US raid in Pakistan.

"The general command of al-Qaida, after completing consultations, declares Abu Mohammed, Ayman al-Zawahri, God help him, the one leading the group," said a statement attributed to al-Qaida and posted on militant websites, including several known to be affiliated with the group.

It gave no details about the selection process but said the choice of al-Zawahri was the best tribute to the memory of the group's "martyears."

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said the US will pursue the new al-Qaida leader just as it did bin Laden.

"As we did both seek to capture and succeed in killing bin Laden, we certainly will do the same thing with Zawahri," he said at a news conference in Washington.

Al-Zawahri, who turns 60 on Sunday and has a $25 million bounty on his head, takes control of al-Qaida at a time when it is struggling to stay relevant in the face of popular uprisings across the Arab world that are demanding Western-style democracy instead of the pan-Islamic nation sought by Islamists.

Still, the lawlessness gripping Yemen, a poor Arabian Peninsula nation, offers al-Qaida a rare opportunity to gain a strategic foothold in the Arab world, bringing it a step closer to the ability to export its extremist brand of Islam to the region.

"He will send his best fighters and organizers there," said Abdel-Rehim Ali, an Egyptian expert on terrorism and extremist Islamic groups. "Yemen is the closest target and a great start for al-Zawahri to realize his dream of an Islamic emirate."

Al-Qaida militants and their allies in Yemen already have taken advantage of the turmoil there to seize control of towns in the south and strike deals with local garrisons to train with weaponry and live openly.

Al-Zawahri, a trained surgeon who hails from an upper-middle-class Cairo family, lacks the populist appeal of his late boss, throwing into doubt whether he would be able to lure young Muslims, particularly in the West, to join al-Qaida's cause.

In Washington, Defense Secretary Robert Gates said al-Zawahri lacks the "peculiar charisma" of bin Laden and said there is suspicion about him among militants because he is Egyptian.

Still, what he lacks in personal magnetism al-Zawahri makes up for with rock solid ideological conviction and organizational and logistical skills, qualities that may have spared al-Qaida a swift demise following its expulsion from Afghanistan in 2001.

It's not clear how much consensus there was over al-Zawahri's succession, but two US officials said he was not a popular choice. They spoke on condition of anonymity to discuss intelligence matters.