Iraqi forces backed by jets and helicopters battled their way to Mosul airport Monday as they prepared to take on the Islamic State group's stronghold in the city's west bank.

The fresh push in the four-month-old operation to retake Mosul has sparked fears for 750,000 trapped civilians who risk getting killed trying to flee and face starvation if they stay.

"The federal police has resumed its advance... Our cannons are targeting Daesh defence lines with heavy fire," federal police chief Raed Shaker Jawdat said in a statement.

Federal police forces, as well as elite units from the interior ministry, army soldiers and Hashed al-Shaabi (Popular Mobilisation) paramilitary fighters, launched a major assault on Sunday.

On the second day of the renewed offensive, they pushed northwards towards Mosul airport, which lies on the southern approach to the city, on the east bank of the Tigris.

The assault marks a new phase in the broad operation launched on October 17 to retake Mosul, Iraq's second city and the jihadists' last major stronghold in the country.

The recapture of Mosul would deal a death blow to the "caliphate" which IS chief Abu Bakr al-Baghdadi proclaimed in the city in June 2014 but has been shrinking steadily for two years as anti-IS forces advanced.

But it took the Iraq's most seasoned forces, the elite Counter-Terrorism Service, two months to retake east Mosul, where IS put up stiffer than expected resistance.

- Pentagon chief in Iraq -

Commanders and experts believe the city's west bank could prove even harder to retake, with the narrow streets of the Old City forcing Iraqi forces to undertake perilous dismounted raids.

IS also "likely has stronger support within western Mosul, and the ISF (Iraqi security forces) are more likely to encounter populations that are wary or hostile," said Patrick Martin, Iraq analyst at the Institute for the Study of War.

Yet after a lull in the four-month-old operation, optimism was high on the front lines.

"I don't expect a very difficult fight: their end has arrived," Hakem Gassem Mohammed, an officer with the interior ministry's Rapid Response force told an AFP reporter south of Mosul.

The first day of the rekindled offensive saw forces advance in sparsely populated areas just south of the city itself.

Elite units trained for urban warfare are expected to move into west Mosul at a later stage.

According to a top army commander coordinating the operation, Abdulamir Yarallah, Iraqi forces on Sunday retook 15 villages on three different fronts converging towards the airport.

The jihadists defending Mosul's west bank have no choice but to defend their bastion. Bridges across the Tigris in the city have been destroyed and Iraqi forces have cut off escape routes.

The forces were receiving support from a US-led coalition that has delivered more than 10,000 munitions on IS targets since the start of the Mosul operation.

- Trapped civilians -

The new Pentagon chief, James Mattis, arrived in Baghdad Monday to show support for the Iraqi security forces, more than 80,000 of whom have also received training from the coalition since 2014.

Before landing in the Iraqi capital, he told reporters that the United States was not about to plunder Iraq's oil reserves.

US President Donald Trump repeatedly said both while campaigning and since his election that America, whose troops occupied Iraqi for years, should have "taken" the oil.

But Mattis, a retired Marine general who commanded troops during the 2003 invasion of Iraq, appeared to nix the idea.

"All of us in America have generally paid for gas and oil all along, and I am sure that we will continue to do so in the future," Mattis said. "We are not in Iraq to seize anybody's oil."

As Iraqi forces advanced on besieged jihadists in Mosul, fears grew for the estimated 750,000 civilian also trapped there with dwindling food supplies.

Save the Children said on Sunday that all parties should protect the estimated 350,000 children among them.

"This is the grim choice for children in western Mosul right now: bombs, crossfire and hunger if they stay -- or execution and snipers if they try to run," said the charity's Iraq director, Maurizio Crivallero.

More than half a million residents stayed home when Iraqi forces entered east Mosul to battle IS there, leading to smaller displacement than initially feared.

But the aid community said the assault on west Mosul could cause a bigger exodus.

"We are racing against the clock to prepare emergency sites south of Mosul to receive displaced families," the UN's humanitarian coordinator in Iraq, Lise Grande, said in a statement.

Iraq launches assault on IS-held west Mosul
Al-Buseif, Iraq (AFP) Feb 19, 2017 - Iraqi forces seized 15 villages from the Islamic State group Sunday, launching a daunting operation to retake west Mosul which aid groups warned will put civilians in grave danger.

Advancing from several directions, the forces moved towards Mosul airport just south of the city, marking a new phase in Iraq's largest military operation in years.

The Islamic State group has put up stiff resistance to defend Mosul, the city where its leader Abu Bakr al-Baghdadi proclaimed a "caliphate" straddling Iraq and Syria in 2014.

A top army commander announced that forces led by federal police units retook villages south of Mosul, including Athbah, leaving them within striking distance of the airport.

"We launched our operation at 7:00 am (0400 GMT)... We are heading towards the airport," said Abbas al-Juburi of the interior ministry's elite Rapid Response force.

The sky south of Mosul was black with smoke from air strikes and artillery as thousands of forces in armoured convoys worked converged on the airport.

"They're desperate," Ali, a Rapid Response officer, said in the village of Al-Buseif as helicopters flew overhead, tracking the last IS fighters attempting to flee.

"They'll try to cause as many losses as possible, because they know they're going to die anyway," his colleague Alaa said.

Hashed al-Shaabi (Popular Mobilisation) paramilitary forces also contributed to the advance on Mosul and to a tally of 15 recaptured villages for the day, according to a statement by top army commander Abdulamir Yarallah.

- Heavier fighting -

The jihadists overran Mosul and swathes of other territory north and west of Baghdad in 2014, routing security forces ill-prepared to face the assault.

The government began the offensive to reconquer Mosul on October 17, throwing tens of thousands of men into the long-awaited counter-attack with air and ground support from a US-led coalition fighting IS in Iraq and Syria.

The Joint Operations Command coordinating the fight against IS declared east Mosul "fully liberated" on January 24.

But it took Iraq's most seasoned forces -- the elite Counter-Terrorism Service -- more than two months to clear the eastern side of Mosul.

After a pause, federal forces now face what was always billed as the toughest nut to crack: Mosul's west bank, home to the narrow streets of the Old City.

"West Mosul had the potential certainly of being more difficult, with house-to-house fighting on a larger and more bloody scale," said Patrick Skinner from the Soufan Group intelligence consultancy.

The coalition said it carried out a total of 40 air strikes on Saturday, nine of which hit targets in the Mosul area.

- US support -

"The US forces continue in the same role they were in in east Mosul and the coalition forces are in support of this operation," US Defence Secretary James Mattis told reporters during a trip to the United Arab Emirates.

More than half of the 9,000-plus coalition forces deployed in Iraq are American, and some were visible on the front line Sunday.

Recent incidents in the recaptured east point to the difficulty of ensuring IS remnants have not blended in with the civilian population.

Aid organisations had feared an exodus of unprecedented proportions before the start of the Mosul operation, but half a million -- a significant majority -- of residents stayed home in east Mosul.

Their continued presence prevented both sides from resorting to deadlier weaponry, which may have slowed down the battle but averted greater displacement and destruction.

The aid community fears a bigger exodus from west Mosul, however.

"We are racing against the clock to prepare emergency sites south of Mosul to receive displaced families," the UN's humanitarian coordinator in Iraq, Lise Grande, said in a statement.

Residents of west Mosul have reported very difficult living conditions and warned that they were already low on food, with weeks of fighting expected ahead.

- Concern for civilians -

Save the Children urged all parties to protect the estimated 350,000 children currently trapped in west Mosul.

"This is the grim choice for children in western Mosul right now: bombs, crossfire and hunger if they stay -- or execution and snipers if they try to run," said the charity's Iraq director, Maurizio Crivallero.

Prime Minister Haider al-Abadi said at the beginning of the year that three more months were needed to rid the country of IS.

That timetable already looks optimistic, but the Norwegian Refugee Council said civilian lives should be the only priority.

"The ultimate success of the offensive will be judged not on how many districts and villages are taken back but on how well Iraqi forces and the US-led coalition protect civilians in the coming weeks and months," NRC's Iraq director Wolfgang Gressmann said.

The recapture of Mosul would effectively end IS's days as a land-holding force in Iraq, with only a pocket around the town of Hawijah and small towns near the Syria border still under its control.

An alliance of Arab and Kurdish forces also backed by the coalition is currently advancing on Raqa in Syria, the only other major hub the jihadists still hold in their now crumbling "caliphate".