Baghdad (AFP) - Prime Minister Nuri al-Maliki remains many Iraqis' top choice to lead the country even though militants have overrun large swathes of territory and his domestic and international support is eroding.

Experts have cast doubt on the incumbent's chances of keeping his post as factions struggle to form a government after April polls, but sectarian allegiance may yet rescue the Shiite Arab leader, and a brutal onslaught by Sunni militants might have even strengthened his supporters' resolve.

A June 13 statement by the country's most senior Shiite cleric Grand Ayatollah Ali al-Sistani, calling on Iraqis to join state forces battling jihadists, has helped rally Iraq's majority Shiites around Maliki and bolstered his image as a bulwark against a perceived Sunni takeover.

"I think support for him has gone up," said student Abbas Saadeq, 21, citing what he saw as backing for the prime minister from Iraq's most senior Shiite clerics, or marjaiya.

"People understand that the marjaiya support the government, so the people support him."

Posters of the heavy-jowled leader still crowd Baghdad's skyline and checkpoints after April's parliamentary election, which his State of Law coalition dominated despite a litany of attacks in the run-up to the poll.

Now, insurgents led by the jihadist Islamic State (IS) group, which is notorious for executing and crucifying opponents, are only about a two-hour drive from the capital's northern borders after a swift advance.

Iraq's army wilted at the beginning of the IS-led onslaught last month, which began in the northern province of Nineveh and spread at lightning speed.

- Accusations against Maliki -

Experts accuse Maliki of micromanaging the military and prioritising fealty and sect over competence, robbing the armed forces of unity and nationalist appeal.

State forces have since bounced back somewhat, albeit with mixed results.

"What has helped Maliki is Daash," said a policeman in Baghdad who declined to be named, referring to IS's Arabic acronym.

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"It has allowed him to rally Shiites behind him. He knows now he can't rely on the army."

The militants' advance was so rapid it prompted the rare call to arms by Sistani, triggering a massive mobilisation of Shiite volunteers and fighters.

Crucially, the octogenarian cleric was careful to specify that any fightback must be under the auspices of the security agencies, infusing Maliki's forces with -- in the eyes of some Iraqis -- religious legitimacy and fighters enthused with sectarian fervour.

Sabah al-Kaabi, a 43-year-old tailor at a barber shop in the capital, said: "What has rescued Maliki is Sistani. If it wasn't for that, Maliki would have had to flee Baghdad."

Some Iraqis are also loathe to see the prime minister go at such a critical time, with many not blaming him for Iraq's misfortunes in any case.

There is no obvious consensus candidate to replace him, or guarantee his successor will not face the same pressures that have contributed to the current political deadlock.

Western countries have stopped short of calling for the premier to go, but Washington in particular has made no secret that it believes he has squandered an opportunity to rebuild the country after American troops left in late-2011.

- 'Never give up' -

Maliki, a dour 64-year-old who spent years in exile in Iran and Syria, rejects this, and said on Friday he would "never give up" on his bid for a third term, citing a popular mandate after a "landslide victory" in April polls.

Not only did his coalition win almost three times as many seats as the next most successful bloc, but Maliki himself also scooped vastly more votes than any other politician.

"I support him. People are against him that won't let him work," said 24-year-old student Ammar al-Abadi, referring to bickering between Sunni Arab, Kurdish and Shiite politicians that has long paralysed parliament.

"We can't just blame him."

Maliki last year reached out to Sunni Arabs, for example, with a pledge to relax a law that had barred former members of Sunni leader Saddam Hussein's Baath Party from public life, but other Shiite lawmakers blocked the move.

Sunni Arabs and Kurd minorities accuse him of sidelining and persecuting them, and of being beholden to powerful Shiite neighbour Iran.

Maliki's supporters counter that Sunni Arabs have never gotten over their reduced status since Saddam's downfall, and accuse the Kurds of exploiting national institutions to create an independent state.

Meanwhile, ordinary Iraqis observing the Ramadan fast in scorching summer heat grimly wait for the country's latest crisis to ease.

"We've lived through the Iran-Iraq war, the UN blockade, two Gulf wars," said Kaabi the tailor, explaining his enduring support for Maliki.

"This is nothing."