Isis jihadists have said they are establishing an Islamic caliphate, extending from Aleppo in northern Syria to Diyala province in eastern Iraq.

The al-Qaeda offshoot, which has taken control of vast swathes of Iraq, has rebranded itself simply the "Islamic State" and named leader Abu Bakr al-Baghdadi the Caliph, or ruler.

In a statement posted on the internet, the militants' spokesman Abu Muhammad al-Adnani said: "He [Baghdadi] is the imam and khalifah [Caliph] for the Muslims everywhere.

"Accordingly, the 'Iraq and Sham' [Levant] in the name of the Islamic State is henceforth removed from all official deliberations and communications, and the official name is the Islamic State from the date of this declaration."

The militants have called upon Muslims everywhere to pledge allegiance to "Caliph Ibrahim", and "reject democracy and other garbage from the West".

The statement reveals the group's plan to extend its control over more territories, saying: "The legality of all emirates, groups, states and organisations becomes null by the expansion of the caliph's authority and the arrival of its troops to their areas.

"Listen to your caliph and obey him. Support your state, which grows every day."

Battle for control of Tikrit

The ultra hardline faction, which sees Shia Muslims as heretics who deserve death, has been disowned by the al-Qaeda leadership and is viewed by Sunni leaders in Gulf Arab states as a long-term security threat.

The rebels have capitalised on the Iraqi Sunnis' deep sense of alienation under the Shiite-led government of Nouri al-Maliki in Baghdad to make huge gains in Iraq, including the strategically vital cities of Baiji, Mosul and Tal Afar.

The battle for control of another key city, Tikrit, has raged on even as the militants appeared to have repelled security forces' efforts to wrest control of key areas.

The Iraqi army said it was in control of Tikrit University, where it raised a government flag. "It is a matter of time before we declare the total clearing of Tikrit," army spokesman Qassim al Moussawi said.

Isis fighters made a lightning advance through northern Iraq early this month, making their long-cherished goal of the creation of a Sunni Islamic caliphate look possible.

The militants ripped through the north of the country, taking control of Mosul and Tikrit and launching an audacious attack on the biggest oil refinery in the country even as the US-trained army disintegrated.

The Sunni insurgency gained in strength following their confiscation of weapons from fleeing Iraqi troops and the looting of millions of dollars from banks in Mosul.

Disgruntled Sunni tribes in pockets throughout the north and west of the country and battle-hardened militants from across the region, especially Syria, also rallied behind them as they cut through Iraqi defences with relative ease.