"He (Zawahiri) and his organisation still threaten us. As we did both seek to capture and kill and succeed in killing bin Laden, we certainly do or will do the same thing with Zawahiri," Admiral Mike Mullen, Chairman of the US Joint Chiefs of Staff, told reporters at a Pentagon news conference.
Mullen said he was not surprised by the news reports that Zawahiri has succeeded Osama bin Laden.
In his last press conference as the Secretary of Defense, Gates said that he is not sure it's a position anybody should aspire to be the al-Qaeda leader, under the circumstances.
"I think he (Zawahiri) will face some challenges," Gates said.
"Bin Laden has been the leader of al-Qaeda, essentially since its inception. In that particular context, he had a peculiar charisma that I think Zawahiri does not have. I think he was much more operationally engaged than we have the sense Zawahiri has been," he said, adding that he has read that there is some suspicion within Al Qaida of Zawahiri because he''s Egyptian.
"I think we should be mindful that this announcement by al-Qaeda reminds us that, despite having suffered a huge loss with the killing of bin Laden -- and a number of others -- al-Qaeda seeks to perpetuate itself, seeks to find replacements for those who have been killed, and remains committed to the agenda that bin Laden put before them," Gates said.
"I think he's got some challenges, but I think it's a reminder that they are still out there, and we still need to keep after it," he said.
"It's probably tough to count votes when you're in a cave," Gates said when asked why it took seven weeks for the al-Qaeda leaders to elect their new leader.
"From my perspective, I don''t take it either way. I think it's just they're working their way through that process. And that's how they made the decision," Mullen said.