With its multi-pronged assault across central and northern Iraq in the past one and a half weeks, the Islamic State of Iraq and the Levant (Isis) has taken over from the al-Qa'ida organisation founded by Osama bin Laden as the most powerful and effective extreme jihadi group in the world.

Isis now controls or can operate with impunity in a great stretch of territory in western Iraq and eastern Syria, making it militarily the most successful jihadi movement ever.

While its exact size is unclear, the group is thought to include thousands of fighters. The last "s" of "Isis" comes from the Arabic word "al-Sham", meaning Levant, Syria or occasionally Damascus, depending on the circumstances.