Prominent Shia leaders pushed Thursday for the removal of Iraqi Prime Minister Nouri al-Maliki as parliament prepared to start work next week on putting together a new government, under intense U.S. pressure to rapidly form a united front against an unrelenting Sunni insurgent onslaught.

Increasingly, the Shia prime minister’s former allies believe he cannot lead an inclusive government that can draw minority Sunnis away from supporting Islamic State of Iraq and Syria (ISIS) fighters who have swept over a large swath of Iraq as they head toward the capital, Baghdad.

In a further sign of Iraq's unravelling along sectarian lines, a bombing on Thursday killed 12 people in a Shia neighbourhood of Baghdad that houses a revered shrine, and police found the bullet-riddled bodies of eight Sunnis south of the capital.

Most crucially, though, backing for al-Maliki is weakening with his most important ally, neighbouring Iran.

A senior Iranian general who met with Shia politicians in Iraq during a 10-day visit this month returned home with a list of potential prime minister candidates for Iran's leadership to consider, several senior Iraqi Shia politicians who have knowledge of the general's meetings told The Associated Press.

Iran's supreme leader, Ayatollah Ali Khamenei, wants al-Maliki to remain in his post, at least for now, the politicians said, but Iran's moderate president, Hassan Rouhani, believes al-Maliki must go or else Iraq will fragment. Khamenei holds final say in all state matters in Iran, but the politicians expressed doubt he would insist on al-Maliki against overwhelming rejection of him by Iraq's Shia parties.

The general, Ghasem Soleimani, is expected to return within days to inform Iraqi politicians of Tehran's favourite, they said, speaking on condition of anonymity to discuss the internal deliberations.

New government an 'urgent priority'

The United States and its allies, meanwhile, are pushing for the creation of a government that can draw support among Iraq's Sunni minority, which has been alienated by al-Maliki.

British Foreign Secretary William Hague, meeting with al-Maliki in Baghdad on Thursday, told a news conference that "we believe the urgent priority must be to form an inclusive government ... that can command the support of all Iraqis and work to stop terrorists and their terrible crimes."

Mideast Iraq

A fleeing Iraqi girl cries after settling with her family near a Kurdish checkpoint between Mosul and Erbil. The United Nations says more than 1,000 people, mainly civilians, have been killed during the Sunni insurgents' advance in Iraq. (Hussein Malla/Associated Press)

Hague's trip follows a visit by U.S Secretary of State John Kerry, who earlier this week delivered a similar message.

Kerry met in Paris on Thursday with foreign ministers from America's top Sunni Arab allies to consider how to confront ISIS.

The threat "concerns every single country here," Kerry told them at the start of the meeting, held at the U.S. ambassador's residence.

The Arab diplomats did not commit to sending any military assistance to Baghdad, as the U.S. is doing.

The Obama administration hopes that Iraq's Sunni neighbours — notably Jordan and Saudi Arabia — will use their cross-border tribal networks to bolster the Sunni militias helping to fight ISIS. However, while they feel threatened by the Islamic State, those Sunni countries are also bitterly opposed to al-Maliki, saying his Shia-dominated rule has marginalized Iraq's Sunnis.

Al-Maliki refuses to step aside

So far, al-Maliki has defied calls to step aside. In April elections, his State of the Law bloc won the largest proportion in parliament — 92 seats in the 328-member chamber — but that is not enough for the simple majority needed to name him prime minister.

He no longer has the support of his former Shia, Kurdish and Sunni allies in his previous coalition.

"We need a government of national consensus. Now, who do you think will not be able to achieve consensus?" said Baleigh Abu Qolal, spokesman for a major Shia party, the Iranian-backed Supreme Islamic Iraqi Council.

Compounding the pressure on al-Maliki, a prominent Shia cleric, Muqtada al-Sadr, called in a televised statement late Wednesday for a national unity government of "new faces" representing all groups.

Al-Sadr, whose followers fought fiercely against both U.S. forces and Sunni extremists during the height of the war nearly a decade ago, also vowed to "shake the ground" under the feet of the Sunni insurgents, who have threatened to advance toward Baghdad and holy Shia cities in the south.

Also, Iraq's most revered and influential Shia cleric, Grand Ayatollah Ali al-Sistani, appealed to al-Maliki through an intermediary to step aside because he fears al-Maliki is driving Iraq into fragmentation, according to a senior member of a prominent Shia family that has for decades maintained regular contact with al-Sistani.

"Al-Sistani is in his 80s and if there is one thing he does not want to see in his lifetime, it is an Iraq breaking up into Sunni,Shia and Kurdish enclaves," the senior family member told the AP.

Al-Sistani, believed to be 86, lives in the Shiite holy city of Najaf, south of Baghdad, where he rarely ventures out of his modest house. But his voice is powerful: His call to arms last week prompted tens of thousands of Shiites to volunteer to fight against the Sunni militants.

Violence continues

And while Iraq's bitterly divided sects focused on self-interests, the country's top Kurdish leader, Massoud Barzani, vowed Thursday to maintain control of the oil-rich city of Kirkuk, seized by Kurdish forces, ostensibly to defend it from the Islamic State fighters.

"We will remain here together in Kirkuk," Barzani declared during a tour of the city, which the Kurds have long sought to incorporate into their self-rule region.

The frequent discovery in recent weeks of bullet-riddled bodies dumped on the streets has raised the spectre of a return of sectarian warfare.

On Thursday, authorities found eight men believed to be Sunnis in their 30s and 40s who had been shot to death in Mahmoudiya, a volatile town 30 kilometres south of Bagdad, police and hospital officials said.

Then, shortly before sunset, a bomb exploded near a clothing shop in Baghdad's northern Shiite neighbourhood of Kazimiyah, killing 12 people and wounding 32, authorities said.