The leader of the so-called Islamic State has called on Muslims worldwide to take up arms and flock to the "caliphate" it had 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.

Baghdadi, who has assumed the mediaeval title of caliph, used the message to seek to assert authority over Muslims everywhere. He called on them to rise up and avenge the alleged wrongs committed against their religion, from Central African Republic to Myanmar (Burma).

"By Allah, we will take revenge, by Allah we will take revenge, even if after a while," he said.

"They have a statement that will cause the world to hear and understand the meaning of terrorism, and boots that will trample the idol of nationalism, destroy the idol of democracy and uncover its deviant nature."

Wednesday's statement comes as Iraq's parliament adjourned for a week, an hour into its first session, after it failed to reach an agreement on senior appointments.

Colonial-era borders

The group, previously known as the Islamic State in Iraq and the Levant (ISIL), proclaimed the caliphate on Sunday and declared Baghdadi its leader, in a bid to sweep away state borders and redraw the map of the Middle East.

Translations in English, Russian, French, German, and Albanian were also posted online.

The declaration of the caliphate followed a three-week drive for territory by ISIL fighters and their allies among Iraqi's Sunni Muslim minority.

Dominating swathes of territory in an arc from Aleppo in Syria to near the western edge of Baghdad, the caliphate aims to erase colonial-era borders and defy the US- and Iranian-backed government of Iraq's Shia prime minister, Nouri al-Maliki.

While ISIL's power grab may appeal to many fighters, there have already been signs of dissent.

Some armed groups fighting in Syria have rejected the announcement of the caliphate, saying its terms had not "been realised at present", and urged Muslims to avoid siding with the Islamic State.

Iraq's Association of Muslim Scholars, which was formed to represent minority Sunnis, said in a statement: "Any group that announces a state or an Islamic emirate ... under these conditions is not in the interest of Iraq and its unity."