In the statement, al-Qaeda "announces that Sheikh Dr Ayman al-Zawahiri, may God guide him, assumed responsibility as the group's amir [leader]".

It warned it would continue to fight a jihad or holy war against the US and Israel under his direction.

The statement posted on a militant website was attributed to al-Qaeda's General Command and was disseminated by the al-Fajr Media Centre, the media wing of al-Qaeda Central.

Egyptian-born Zawahiri was for years Bin Laden's deputy and had been widely anticipated to replace him at the helm.

Zawahiri, 60, is claimed by some experts to have been the "operational brains" behind the 9/11 attacks on the US. He warned just over a week ago that Bin Laden would continue to "terrify" the US from beyond the grave.

In a video message posted on the internet on June 8, Zawahiri said al-Qaeda would continue to fight.

Zawahiri has for years had a bounty on his head and security analysts have suggested he is most likely to be hiding in the Afghan-Pakistan border region.

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.

Bin Laden's killing by US special forces in a covert operation in the garrison town of Abbottabad on May 2 strained Washington's relations with Islamabad.