Adm Mike Mullen said al-Qaeda still posed a threat to the US. "As we did both seek to capture and kill - and succeed in killing - Bin Laden, we certainly will do the same thing with Zawahiri," he said.

Zawahiri's appointment 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," Adm Mullen, chairman of the Joint Chiefs of Staff, told journalists.

Zawahiri has for a long time been the militant group's second-in-command. Osama Bin Laden was killed by US special forces in Pakistan in May.

However, Bin Laden and other key militant leaders who were also believed to be concealed there have instead been discovered in Pakistani towns and cities.