Al Qaeda's new chief Ayman al-Zawahri lacks Osama bin Laden's stature among Islamists worldwide but the United States is just as determined to hunt him down and kill him as it did his predecessor, U.S. officials said on Thursday.

Defense Secretary Robert Gates said Zawahri, al Qaeda's longtime second-in-command and now its top leader, does not have the "peculiar charisma" and operational experience of bin Laden, who was killed by U.S. forces last month.

But Gates and other U.S. officials said al Qaeda remains a threat despite its loss of bin Laden, who was considered the driving force behind the Sept. 11, 2001, attacks on the United States.

"We should be mindful that ... al Qaeda seeks to perpetuate itself, seeks to find replacements to those that have been killed and remains committed to the agenda that bin Laden put before them," Gates told reporters.

"So I think he's (Zawahri's) got some challenges but I think it's a reminder that they are still out there and we still need to keep after them," he said.

Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, made clear that Zawahri -- an Egyptian-born ideologue -- remained high on the U.S. list of hunted militants even after commandos killed bin Laden in a raid on his Pakistan hide-out 45 days ago.

"He and his organization still threaten us. And as we did seek to capture and kill -- and succeed in killing -- bin Laden, we certainly will do the same thing with Zawahri," Mullen told reporters.

Zawahri has taken over the leadership after the killing of bin Laden, the group said on Islamist websites on Thursday.

'NO SURPRISE'

The White House said Zawahri's rise had been expected since he had long served as bin Laden's deputy, but the State Department said it "barely matters" who the new leader was, contending the violent Islamist group's influence was on the wane.

"It's neither surprising nor does it change some fundamental facts," White House spokesman Jay Carney told reporters. "Al Qaeda's ideology is bankrupt. The fact is the peaceful movements for change are the future of the region and al Qaeda is the past."

U.S. officials, in a rhetorical campaign that seemed designed to undercut the new al Qaeda leader, also raised doubts about whether Zawahri had the personality to emulate the unifying role played by bin Laden. Bin Laden was the network's founding figurehead and became a global symbol of Islamist militancy despised in the West but admired by some in Muslim countries.

"He hasn't demonstrated strong leadership or organizational skills during his time in AQ," a senior administration official said. "His ascension to the top leadership spot will likely generate criticism if not alienation and dissension (within al Qaeda)."

"Unlike many of AQ's top members, Zawahri has not had actual combat experience, instead opting to be an armchair general with a 'soft' image," the official said, speaking on condition of anonymity.

Gates said he understood there was also some suspicion among militants because Zawahri is Egyptian.

Zawahri's apparently prickly temperament and Egyptian background could make it hard to mediate between the Egyptians who have dominated the upper reaches of the central al Qaeda group and other militants, including nationals of Arab, Asian, African and European countries as well as of the United States, experts say.

The administration official said the Zawahri would also have a hard time leading al Qaeda because he would have to focus "on his own survival."

Asked at a White House briefing whether the United States had any plans to send Zawahri "a congratulatory drone or bunker buster," Carney said tersely: "I have no comment on that."