BAGHDAD (REUTERS) - A column of Shi'ite militia fighters arrived at a military base near Ramadi on Monday as Baghdad moved to retake the western Iraqi city that fell to Islamic State in Iraq and Syria (ISIS) militants in the biggest defeat for the government since last summer.

The US-led coalition stepped up air raids against the Islamists, conducting 19 strikes near Ramadi over the past 72 hours at the request of the Iraqi security forces, a coalition spokesman said.

The militia, known as Hashid Shaabi or Popular Mobilisation, had been ordered to mobilise after the city, the capital of Anbar province, was overrun on Sunday.

The militiamen give the government far more capability to launch a counterattack, but their arrival could add to sectarian animosity in one of the most violent parts of Iraq. "Hashid Shaabi forces reached the Habbaniya base and are now on standby," said the head of the Anbar provincial council, Sabah Karhout.

An eyewitness described a long line of armoured vehicles and trucks mounted with machine guns and rockets, flying the yellow flags of Kataib Hezbollah, one of the militia factions, heading towards the base.

Spokesmen for militia groups said reconnaissance and planning were underway for the upcoming "battle of Anbar", the vast Euphrates River valley province where the US military fought the biggest battles of its 11-year occupation.

Ramadi is dominated by Sunni Muslims. Prime Minister Haider al-Abadi signed off on the deployment of Shi'ite militias to attempt to seize back the area, a move he had previously resisted for fear of provoking a sectarian backlash.

About 500 people have been killed in the fighting for Ramadi in recent days and between 6,000 and 8,000 have fled, a spokesman for the provincial governor said.

The city's fall marked a major setback for the forces ranged against ISIS: the U.S.-led coalition and the Iraqi security forces, which have been propped up by Iranian-backed Shi'ite militias It was also a harsh return to reality for Washington, which at the weekend had mounted a successful special forces raid in Syria in which it said it killed an ISIS leader in charge of the group's black market oil and gas sales, and captured his wife.

The Iraqi government and Shi'ite paramilitaries recaptured former dictator Saddam Hussein's Tigris River home city of Tikrit from ISIS six weeks ago, the biggest advance since the militants swept through northern Iraq last year.

But government forces have had less success in the valley of Iraq's other great river, the Euphrates, west of Baghdad.

Dozens of apostates

ISIS said that in Ramadi it had seized tanks and killed "dozens of apostates", its description for members of the Iraqi security forces.

An eyewitness in Ramadi said bodies of policemen and soldiers lay in almost every street, with burnt-out military vehicles nearby. Every building was pocked with bullet holes.

An army major who fought his way out of Ramadi said government forces in the area had been ordered to regroup, but soldiers were exhausted and morale was at rock bottom.

Taking Ramadi was the biggest victory for ISIS in Iraq since security forces and Shi'ite paramilitaries began pushing the militants back last year, aided by coalition air strikes.

But analysts believe the coalition strategy of air raids against ISIS is failing to break a group that has shown a great deal of resilience, while it was clear for some time that Iraqi security forces in Ramadi were not up to the job. "It's a wake-up call that the current Americans strategy in Iraq is not working," said Hassan Hassan, author of a book on ISIS.

"The Americans said that they have carried out air strikes against ISIS but then the group went in and defeated the local forces. So they really need to come up with a whole new strategy... and really take the fight to them." While the government in Baghdad has urged Sunni tribes in Anbar to accept help from Shi'ite militia against ISIS, many Sunnis regard the Shi'ite fighters with deep hostility.

ISIS portrays itself as a defender of Sunnis against the Iran-backed Shi'ite fighters. In an example of the sectarian mistrust, an Anbar Sunni tribal leader now in exile in the Kurdish regional capital of Erbil said the deployment of the Hashid Shaabi showed that Baghdad's goal was to crush Sunnis.

Describing Anbar as the stronghold of Sunnis in Iraq, Sheikh Ali Hamad said: "They wanted to destroy this citadel and break its walls so that the Hashid could enter in order to spread Shi'ism."

But some tribes are so fearful of ISIS's harsh rule that they may be willing to accept a role even for the hated Shi'ite militias. Another tribal leader, Sheikh Abu Majid al-Zoyan, said he was suspicious of the Shi'ite militias, but"at this stage, we welcome any force that will come and liberate us from the chokehold" of ISIS.

Kerry confident

United States Secretary of State John Kerry expressed confidence that the takeover of Ramadi would be reversed in the coming weeks.

Ali Akbar Velayati, a senior Iranian official, said Tehran was ready to help confront ISIS, and he was certain the city would be "liberated" from their grip.

US officials say they believe Iran, one of Iraq's main allies, has carried out air strikes against ISIS in Iraq.

The Iraqi government had vowed to liberate Anbar after routing the militants from Tikrit. But the security forces, which partly disintegrated under an ISIS onslaught last June, have struggled to make progress in the vast province.

An officer who withdrew from the besieged army base said the militants - known in Arabic as Daesh - were urging them via loudspeaker to discard their weapons in return for mercy. "Most of the troops withdrew from the operations command headquarters and Daesh fighters managed to break in from the southern gate," the officer said. "We are retreating to the west to reach a safe area".

Before it fell, Ramadi was one of only a few towns and cities still under government control in Anbar, which borders Saudi Arabia, Syria and Jordan. ISIS, which emerged as an offshoot of al Qaeda, controls large parts of Iraq and Syria in a self-proclaimed caliphate where it has carried out mass killings of members of religious minorities and beheaded hostages.

A senior Israeli intelligence official said that before US-led coalition forces began operations against the group, its revenues were running at about $65 million a month, more than 90 percent of which came from its oil business and the rest from locally imposed taxes and ransom money.

Since then, monthly revenues had fallen to about US$20 million, of which about 70 per cent is from oil and the rest from taxes and ransom.