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.