Washington's top military officer had his own announcement, following the one that declared Ayman al-Zawahiri, Osama bin Laden's longtime deputy, new al-Qaeda chief: We will hunt you down and kill you, too, BBC News reported.

As we did both seek to capture and kill - and succeed in killing - bin Laden, we certainly will do the same thing with Zawahiri, said Admiral Mike Mullen.

Zawahiri's new job was posted on a militant website and attributed to al-Qaeda's General Command.

There is not a surprise, from my perspective, that he has moved into that position, Mullen, chairman of the Joint Chiefs of Staff, said.

He and his organization are still threatening us.

And U.S. Defense Secretary Robert Gates agreed.

First of all, I think we should be mindful that this announcement by al-Qaeda reminds us that despite having suffered a huge loss... 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.

Despite being al-Qaeda's second-in-command for a long while, he lacks some of the qualities of bin Laden as a terror leader, Gates said.

Bin Laden has been the leader of al-Qaeda essentially since its inception, he said.

In that particular context, he had a peculiar charisma that I think Zawahiri does not have. I think he has much more operationally engaged than we have the sense Zawahiri has been.

Like bin Laden, Zawahiri has been in hiding since the United States declared its war on terror after the Sept. 11, 2001 attacks.

Zawahiri, who's turning 60 this month, is a surgeon from a prominent Egyptian family. He worked with Osama bin Laden for decades and is credited for al Qaeda's use of bombings and independent terror cells.

Zawahiri is now the United States' most wanted man.