The holy Shiite city of Najaf has gained prominence as a centre of political and military power since the start of a crisis that has raised the spectre of Iraq breaking up along sectarian and ethnic lines.

Most of Iraq's political leaders say they want to preserve national unity, yet a Sunni jihadist offensive has seen influence ebb from Baghdad and flow to Iraq's north, where Kurds aim to form an independent state, and to the southern city of Najaf, home to Shiite-majority's Iraq's holiest shrines and most powerful clerics.

While the central government has struggled to respond to the Islamic State (IS) onslaught, which began on June 9, Najaf has absorbed thousands of Iraqis fleeing the violence, and its clerics have rallied thousands of Shiite fighters to bolster a flagging national army.

"Thank God for these noble people, these merciful people," said Abu Hussein, 36, a Turkmen Shiite who has been housed in a prayer hall in Najaf along with 25 relatives after fleeing fighting further north.

"They gave us everything... rice, oil, meat, clothes, nappies," he said with tears in his eyes, describing how he had sold his wife's and sisters' jewellery to raise money to move his family south.

Wracked by violence stoked by political instability, Baghdad is not high on the list of any Iraqi seeking safety, with many fleeing to Kurdish-controlled areas or the relatively stable Shiite south.

Shortly after arriving in Najaf, Abu Hussein said he had received 50,000 dinars ($43, 32 euros) per family member from a charity connected to Iraq's most revered Shiite cleric, Najaf-based Grand Ayatollah Ali al-Sistani.

- Call to arms -

Najaf governorate, of which Najaf is the capital city, in 2007 had an estimated population of just 615,000, yet it has hosted at least 24,000 displaced people since the IS offensive began.

The Iraqi Red Crescent said their treatment had been "outstanding".

The city's golden-domed Imam Ali shrine, one of the holiest in Shiite Islam, is distributing around 9,000 meals a day to the needy, and not just to Shiites -- some Sunnis are sheltering in the city, too.

Adding to the sense of the safety is the mobilisation of thousands of Shiite volunteers by a recent fatwa from Sistani calling for jihad against IS, the first such edict in Iraq more than 90 years.

Some 5,000 Shiite men have volunteered to take up arms to protect the shrine and its environs alone, a prize target for Sunni extremists who consider Shiites heretics.

Thousands more have gone to bolster a national army that folded in the initial stages of the IS offensive, which began in the northern city of Mosul before spreading to other northern and western provinces.

"We've had so many applications, I've had to turn people away. Where are we going to put them all?" said Sayed Shubr, 42, an expert in Chinese medicine and former member of the Iraqi army who now trains new recruits.

His company operates under the auspices of the national security forces and is entitled "In Service of Imam Hussein", part of a wider volunteer group that also includes a "martyrdom" unit for the most dangerous missions.

- Clerics strident -

Sistani's fatwa was one of a string of recent pronouncements suggesting a more strident role for Iraq's top Shiite clerics, collectively known as the marjaiya, at least while Baghdad founders.

Parliament failed even to muster enough lawmakers to hold an emergency session in the days following the IS onslaught, or agree a new government after April elections.

In a June 20 statement, Sistani called for the "formation of an effective government that is acceptable on a ... national level (and) avoids past mistakes" in a message seen as a rebuke to Prime Minister Nuri al-Maliki, considered by many a divisive and sectarian leader.

Sistani's interventions were unusually robust for a clergyman keen to stay aloof from Iraq's graft-ridden politics and wary of being seen to meddle.

But in times of crisis, the Najaf-based marjaiya have no choice but to step forward, explained the son of, and spokesman for, Grand Ayatollah Bashir al-Najafi, another senior Shiite cleric.

"The new Iraq has its own situation and character, and those who drew Iraq's new map are the marjaiya," Sheikh Ali al-Najafi told AFP, referring to Iraq after the 2003 US invasion, when the marjaiya insisted on elections and democratic institutions.

"The marjaiya does not want to control the country itself. But in times of emergency, and in difficult, critical times, they express an opinion."