The formation of a new government in Iraq has suffered a setback with the biggest bloc in parliament choosing to abstain from an upcoming parliamentary session, even as the military continued its offensive against the Sunni rebels for the second day.

Iyad Allawi, Iraq's former prime minister, on Sunday called on his supporters to boycott a crucial parliamentary vote, as world leaders urged the speeding up of government formation amid violent unrest.

Iraq's National Coalition led by Allawi has demanded the creation of a "road map to stop the security deterioration and heal country's rift". Politicians from the 21-seat bloc said in a statement that more time was needed to avoid the mistakes of the last government.

"The National Coalition has decided not to attend the first session of parliament, to distance ourselves from the consequences of the session, unless the political powers put a road map to stop the security deterioration and save the country," said Hatim Hayeb, a member of the National Coalition.

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Reuters news agency reported that top Shia, Sunni and Kurdish lawmakers scrambled to agree cabinet nominations before the parliament meeting on Tuesday, to try to prevent the rebel advance threatening Iraq's future as a unitary state.

Baghdad took delivery of Russian warplanes to aid it in a crisis said to rival the brutal sectarian war of 2006-2007, with more than 1,000 killed and hundreds of thousands displaced within weeks.

World leaders have insisted on a political settlement among Iraq's various communities and Grand Ayatollah Ali al-Sistani, revered among the country's Shia majority, has urged political leaders to quickly form a government.

Establishment of caliphate

Meanwhile, fighters from the Islamic State of Iraq and the Levant (ISIL) have announced the establishment of a "caliphate".

In an audio recording distributed online on Sunday, ISIL declared its chief Abu Bakr al-Baghdadi as "the caliph" and "leader for Muslims everywhere".

Prime Minister al-Maliki is under intense pressure as Sunni fighters led by the ISIL consolidate their grip on the north and the west of the country.

The Iraqi government continued its biggest push yet to drive back the rebel offensive on Sunday, as soldiers backed by tanks and helicopter gunships began an offensive to retake Tikrit.

Iraqi soldiers said they uncovered an ISIL weapons storage facility and regained two trucks that had been taken by rebels.

There were conflicting reports as to just how much headway the Iraqi military made in its initial thrust towards Tikrit, the hometown of Saddam Hussein, according to the AP news agency.

Residents said Sunni fighters were still in control of the city during the night, but Iraqi officials said the troops had reached the outskirts and even entered Tikrit itself.

Source: Al Jazeera and agencies