Maliki's bid for a third term in office has been battered by the offensive and he is no longer seen as the clear front runner when parliament reopens today following elections in April.

IS announced on Sunday it was establishing a "caliphate" - an Islamic form of government last seen under the Ottoman Empire - extending now from Aleppo in northern Syria to Diyala province in eastern Iraq.

In an audio recording distributed online, the group declared its chief Abu Bakr al-Baghdadi (pictured) "the caliph" and "leader for Muslims everywhere". The group said he was to be known as Caliph Ibrahim - a reference to his real name.

Though the move may not have immediate significant impact on the ground, it is an indicator of the group's confidence and marks a move against al-Qaeda - from which it broke away - in particular, analysts say.

The caliphate was "the biggest development in international jihad since September 11", said Charles Lister of the Brookings Institution in Doha.

"It could mark the birth of a new era of transnational jihadism … and that poses a real danger to al-Qaeda and its leadership," he said, adding that IS, with members in many countries, was the richest jihadist group.

Baghdadi, thought to have been born in the Iraqi city of Samarra in 1971, is touted by the group as a battle-hardened tactician who fought American forces following the 2003 US-led invasion of Iraq, and is now widely seen as rivalling al-Qaeda chief Ayman al-Zawahiri as the world's most influential jihadi. His group has drawn thousands of foreign fighters, attracted by a combination of Baghdadi's own appeal, IS efforts to establish what it believes is an ideal Islamic state, and the group's sophisticated propaganda apparatus which publishes magazines and videos in English and a number of European languages.

The group is known for its brutality, executing its opponents and this week crucifying rival Islamic rebels in Syria.

Since the Prophet Mohammed's death, a caliph was designated "the prince" or emir "of the believers".

In Syria, IS fighters control large swathes of territory in Deir Ezzor near the Iraqi border, Raqa in the north, as well as parts of neighbouring Aleppo province.

In Iraq, it has spearheaded a lightning offensive since June 9, capturing sizeable territories in the north and west.

Iraqi forces initially wilted in the face of the onslaught but have mounted an ambitious counteroffensive to take back Tikrit.

A security source based north of the city said reinforcements had arrived with tanks and artillery, with an army officer saying the Iraqi military controlled parts of the outskirts of the city.

Witnesses reported air strikes on Tikrit yesterday, while clashes were reported in several areas around the city on Sunday.