A long letter purportedly from Osama bin Laden's deputy, made public this week by the US, has been interpreted by some as a fundamental shift of strategy by the leadership of al-Qaeda. But it has also generated debate among experts about whether the letter is genuine - and, if it is, whether the US has mistakenly identified its intended recipient.

The US said the letter was from Ayman al-Zawahiri, al-Qaeda's number two, who is apparently holed up in the tribal areas of Pakistan. It said it was sent to Abu Musab al-Zarqawi, Jordanian-born head of a high- profile terror group called al-Qaeda in Iraq.