The wife of al Qaeda leader Ayman al-Zawahri praised Muslim women for their role in the Arab Spring uprisings and said the unrest would soon lead to an "Islamic Spring", according to a rare message posted online on Friday.

The letter, signed by Omaima Hassan, singled out women beaten during Egypt's unrest and lauded mothers for bringing up the revolutionaries who went on to topple four heads of state it described as "tyrant criminals".

It was not immediately possible to verify the authenticity of the undated message, posted on a website used by Islamist militants.