Iraq's army sent tanks and armoured vehicles to try to dislodge insurgents from the northern city of Tikrit on Sunday, the second day of a pushback against a Sunni militant takeover of large stretches of Iraq.

The hardline Sunni group leading the insurgency, until Sunday known as the Islamic State in Iraq and the Levant (ISIL), declared itself a "caliphate" on Sunday and called on factions worldwide to pledge their allegiance - a move analysts saw as a direct challenge to al Qaeda, which disowned ISIL in February, and to Gulf rulers.

In Baghdad, which is threatened by the rebel advance, top Shi'ite, Sunni and Kurdish lawmakers scrambled to agree cabinet nominations before parliament meets on Tuesday to try to prevent the rebel advance jeopardising Iraq's future as a unitary state.

They are racing against time as ISIL, which loathes Prime Minister Nuri al-Maliki's Shi'ite-led government, consolidates its grip on the north and west. Maliki's political future after eight years in power will be the most contentious issue.

Troops backed by helicopter gunships began an assault on Tikrit, the birthplace of former President Saddam Hussein, on Saturday, to try to take it back from insurgents who have swept to within driving range of Baghdad.

The army sent in tanks and helicopters to battle ISIL militants near the University of Tikrit in the city's north on Sunday, security sources said. Two witnesses said they saw a helicopter gunned down over northern Tikrit, reports not possible to immediately verify independently.

The offensive was the first major attempt by the army to retake territory after the United States sent up to 300 advisers, mostly special forces, and drones to help the government take on ISIL.

Earlier on Sunday, Deputy Prime Minister for Energy Hussain al-Shahristani, one of Iraq's most senior politicians, faulted the US for not doing enough to bolster the country's military, just hours after Russia delivered five Sukhoi jets.

"Yes, there has been a delay from the Americans in handing over contracted arms. We told them, 'You once did an air bridge to send arms to your ally Israel, so why don't you give us the contracted arms in time?'" he told al-Hurra television.

U.S. officials have disputed similar statements from Iraqi officials in the past and say they have done everything possible to ensure the country is equipped with modern weaponry.

The five Russian Sukhoi jets were delivered to Baghdad late on Saturday. State television said they "would be used in the coming days to strike ISIL terrorist groups".

A Reuters photographer saw the jets unloaded from a transport plane at a military airport in Baghdad as Russian and Iraqi soldiers stood on the tarmac. Iraq has relied largely on helicopters to counter militants and has few aircraft that can fire advanced missiles.

Iraqi army spokesman Qassim Atta told reporters in Baghdad security forces had killed 142 "terrorists" over the last 24 hours across Iraq, including 70 in Tikrit, and said the armed forces were in control of Tikrit's university. Both claims were impossible to immediately verify.

"Our security forces have taken complete control of the University of Tikrit and they have raised the Iraqi flag on top of the building," Atta said.

Iran has also supported Iraq's government against the onslaught. An Iranian general said on Sunday his country was ready to help Iraq fight the revolt using the same methods it deployed against rebels in Syria.

"With Syria, too, we announced we would not allow terrorists in the hire of foreign intelligence services to rule and dictate to Syrian people. We will certainly have the same approach with Iraq," Brigadier General Massoud Jazayeri, deputy joint chief of staff of the armed forces and a senior Islamic Revolutionary Guards Corps officer, told Iran's al Alam television.

On Sunday, intermittent clashes broke out from the early morning between militants and government forces in the northeastern outskirts of the town of Jurf al-Sakhar, 53 miles(83 km) south of Baghdad.

The local government and security commanders have asked for backup from Baghdad to face what they estimate are several hundred ISIL fighters, police sources and the province's governor said.

In a statement distributed on Islamist forums, ISIL, also known as ISIS, said it was renaming itself "Islamic State" and proclaiming its leader Abu Bakr al-Baghadi "caliph" of the territory it has seized in Iraq and Syria.

"He is the imam and khalifah (caliph) for the Muslims everywhere," the group's spokesman Abu Muhammad al-Adnani said in the statement, which was translated into several languages and an Arabic audio speech.

ISIL and allied militias have seized funds, equipment and control of border posts, oilfields and swathes of territory including northern Iraq's largest city Mosul throughout its nearly three-week-old offensive.

ISIL has long vowed to re-create a medieval-style caliphate erasing borders from the Mediterranean to the Gulf and they deem all Shi'ites to be heretics deserving death.

In Syria, ISIL fighters crucified eight men in the northern Aleppo province, a monitoring group said. ISIL accused them of being "Sahwa" fighters, a term it uses for rival fighters it says are controlled by Western powers.

The men were crucified in the town square of Deir Hafer in eastern Aleppo and would be left there for three days, the Britain-based Syrian Observatory for Human Rights said.

Politicians are under pressure to speed up the normally sluggish process of selecting a new government to face the crisis. A parliament elected in April is due to assemble on Tuesday to begin the process.

In a statement on Sunday, the United Nations mission in Iraq urged all representatives to attend the session on Tuesday and move forward with selecting a new government.

"Faced with a national crisis, the political leaders of Iraq should put the interests of the country and its people before everything else," Special Representative of the U.N. Secretary-General for Iraq Nickolay Mladenov said in the statement.

But the 21-seat bloc of former prime minister Iyad Allawi, a secular Shi'ite, said it would skip the session, arguing more time was needed to avoid the previous government's mistakes.

Politicians from the National Alliance, parliament's biggest bloc, said they would join the session and seek to follow the timetable for the formation, but were tight-lipped about who they would back for prime minister. A senior member of the Patriotic Union of Kurdistan, Adnan Mufti, said it would attend.-Reuters