The world's most wanted terror leader, Osama bin Laden, is alive and well, his al Qaeda number two has asserted.

Bin Laden has claimed responsibility for the September 11 attacks on the United States in 2001, but has so far evaded all attempts to capture him.

In an audio message monitored by a US group keeping track of Islamist websites, Ayman al Zawahiri declared: "Sheikh Osama bin Laden is healthy and well by the grace of Allah."