EAST OF MOSUL/BAGHDAD (Reuters) - Iraqi government forces launched a U.S.-backed offensive on Monday to drive Islamic State from the northern city of Mosul, a high-stakes battle to retake the militants’ last major stronghold in the country.

Two years after the jihadists seized the city of 1.5 million people and declared a caliphate from there encompassing tracts of Iraq and Syria, a force of some 30,000 Iraqi and Kurdish Peshmerga forces and Sunni tribal fighters began to advance.

Helicopters released flares and explosions could be heard on the city’s eastern front, where Reuters watched Kurdish fighters move forward to take outlying villages.

A U.S.-led air campaign has helped push Islamic State from much of the territory it held but 4,000 to 8,000 fighters are thought to remain in Mosul.

The Pentagon said that Iraqi forces were meeting objectives and were ahead of schedule on the first day of the offensive.

Residents contacted by phone dismissed reports on Arabic television channels of an exodus by the jihadists, who have a history of using human shields and have threatened to unleash chemical weapons.

“Daesh are using motorcycles for their patrols to evade air detection, with pillion passengers using binoculars to check out buildings and streets,” said Abu Maher, using an Arabic acronym for Islamic State.

He and others contacted were preparing makeshift defenses and had been stockpiling food in anticipation of the assault, which officials say could take weeks or even months. The residents withheld their full names for security reasons and Reuters was not able to verify their accounts independently.

The United States predicted Islamic State would suffer “a lasting defeat” as Iraqi forces mounted their biggest operation in Iraq since the 2003 invasion that toppled Saddam Hussein.

But the offensive, which has assumed considerable importance for U.S. President Barack Obama as his term draws to a close, is fraught with risks.

These include sectarian conflict between Mosul’s mainly Sunni population and advancing Shi’ite forces, and the potential for up to a million people to flee Mosul, multiplying a refugee crisis in the region and across Europe.

“We set up a fortified room in the house by putting sandbags to block the only window and we removed everything dangerous or flammable,” Abu Maher said. “I spent almost all my money on buying food, baby milk and anything we might need.”

The United Nations’ humanitarian coordinator for Iraq said the military had told the U.N. it expected the first significant population movement to begin in five to six days, suggesting that is when the assault would move to the city itself.

Lise Grande said Iraqi security forces would transport fleeing civilians, who would be vetted to ensure Islamic State fighters could not hide among them, following residents’ reports that militants had shaved off their beards to escape detection.

Video showing rockets and bursts of tracer bullets across the night sky and loud bursts of gunfire was shown on Qatar-based al-Jazeera television after Prime Minister Haider Abadi announced what he called “the heroic operations to free you from the terror and oppression of Daesh”.

“We will meet soon on the ground in Mosul to celebrate liberation and your salvation,” Abadi said in a speech on state television in the middle of the night, surrounded by commanders of the armed forces.

KURDISH FIGHTERS

The commander of the coalition, U.S. Lieutenant General Stephen Townsend, said the operation to take Iraq’s second largest city would likely continue for weeks, “possibly longer”.

The Pentagon played down any new role for U.S. forces in the battle saying American personnel were behind the forward line of troops and acting in an advisory role to support Iraqis.

“Americans are again playing an advisor role, an enabler role for these Iraqi forces ... Most of the American forces in Iraq are not anywhere close to the front line,” Pentagon spokesman Peter Cook told a news briefing, saying many U.S. troops were on advisory or logistical support missions.

If Mosul falls, Raqqa in Syria will be Islamic State’s last city stronghold.

“This is a decisive moment in the campaign to deliver ISIL a lasting defeat,” U.S. Defense Secretary Ash Carter said in a statement, using another acronym for Islamic State.

“We are confident our Iraqi partners will prevail against our common enemy and free Mosul and the rest of Iraq from ISIL’s hatred and brutality.”

Islamic State has been retreating since the end of last year in Iraq, where it is battling U.S-backed government and Kurdish forces as well as Iranian-backed Iraqi Shi’ite militias.

But in a blow to the anti-IS campaign, last week the jihadists crushed a planned rebellion in Mosul intended to ease the recapture of the city.

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The Iraqi Kurdish military command said 4,000 Peshmerga were taking part in an operation to clear villages held by Islamic State to the east of Mosul, in an attack coordinated with a push by Iraqi army units from the southern front.

Massoud Barzani, president of Iraq’s Kurdistan region, said the operation was the first time the two forces had fought against Islamic State together and had taken about 200 square km of farm land and villages east of Mosul from the militants.

The IS-linked Amaq news agency said the group had launched 10 suicide attacks on advancing forces near Mosul and that Peshmerga forces had encircled five villages but failed to take them. None of the reports could be independently verified.

A column of black smoke was rising from one IS position on the eastern front, a Reuters correspondent said, apparently from burning oil being used to block the path of the Kurds and obscure the jets’ view.

“We are the real Muslims, Daesh are not Muslims, no religion does what they did,” said a young Kurdish fighter in battle dress as he scanned the plain east of Mosul from his position on the heights of Mount Zertik.

As he spoke a Humvee drove by with the word Rojava, or Syria’s Kurdistan, painted on the machine gun turret.

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“This is all Kurdistan,” Major Shiban Saleh, one of the fighters onboard, said. “When we’re done here, we will chase them to Raqqa or wherever they go,” he said.

He said about 450 Syrian Peshmerga fighters were involved in the offensive, which aimed to take back nine villages on Monday.

Abdul Rahman Waggaa, a member of the exiled Provincial Council of Nineveh of which Mosul is the capital, told Reuters the advancing forces had yet to enter the city: “Activities are still outside of Mosul and the operation is not at full strength yet.”

HUMANITARIAN CRISIS FEARED

Early on Monday, Abadi sought to allay fears that the operation would provoke sectarian bloodletting, saying that only the Iraqi army and police would be allowed to enter the mainly Sunni city. He asked Mosul’s residents to cooperate with them.

Local Sunni politicians and regional Sunni-majority states including Turkey and Saudi Arabia warned that if Shi’ite militias take part in the assault they could spark sectarian violence.

The Iraqi army dropped tens of thousands of leaflets on Mosul before dawn on Sunday, warning residents the offensive was imminent, assuring them it “will not target civilians” and telling them to avoid known locations of Islamic State fighters.

Reflecting authorities’ concerns over a mass exodus that would complicate the offensive and worsen the humanitarian situation, the leaflets told residents “to stay at home and not to believe rumors spread by Daesh” that could cause panic.

Resident Abu Abdullah said he had wanted to witness the beginning of the offensive.

“We heard repeated explosions at a distance, so I went to the rooftop to see fireballs, even if it was dangerous. I was happy that the operation to liberate Mosul started,” he said.

In 2014, Islamic State leader Abu Bakr al-Baghdadi proclaimed a “caliphate” in Iraq and neighboring Syria from Mosul’s Grand Mosque. The group faced little resistance but has employed brutal methods to maintain control. On Monday, it circulated photographs showing children executing alleged spies.

Russian President Vladimir Putin, criticized over the level of civilian casualties during Syrian government operations backed by Moscow in and around the city of Aleppo, said on Sunday he hoped the United States and its allies would do their best to avoid hitting civilians in the attack on Mosul.

The United Nations has said the battle would require the world’s biggest and most complex humanitarian effort, which could leave up to 1 million people homeless and see civilians used as human shields or even gassed.

There are already more than three million people displaced in Iraq as a result of conflicts involving Islamic State and up to 100,000 Iraqis may flee Mosul to Syria and Turkey. Medicine is in short supply in Mosul and food prices have risen sharply.

“Families in Mosul started stockpiling food yesterday in case the fighting reaches our streets and we can no longer go out,” said Saeed, a resident.

“Daesh are still in Mosul and it’s not true that they left. They are continuing to erect blast walls in the streets to obstruct any advance.”