(Reuters) - The leader of the al Qaeda offshoot now calling itself the Islamic State has called on Muslims worldwide to take up arms and flock to the "caliphate" it has declared on captured Syrian and Iraqi soil.

Proclaiming a "new era" in which Muslims will ultimately triumph, Abu Bakr al-Baghdadi issued the call to jihad - holy war - in an audio message lasting nearly 20 minutes that was posted online on Tuesday.

It was his first purported message since the group - previously known as the Islamic State in Iraq and the Levant (ISIL) - proclaimed the caliphate on Sunday and declared him its leader, in an audacious bid to sweep away state borders and redraw the map of the Middle East.

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