Howitzer and mortar fire started at dawn, hitting a group of villages held by Islamic State about 10-20 km from Mosul, while helicopters flew overhead, according to Reuters reporters at two frontline locations north and east of Mosul.

To the sound of machine gun fire and explosions, dozens of black Humvees of the elite Counter Terrorism Service (CTS), mounted with machine guns, headed towards Bartella, an abandoned Christian village just east of Mosul.

Militants were using suicide car-bombs, roadside bombs and snipers to resist the attack, and were pounding surrounding areas with mortars, a CTS commander said.

Hours later, the head of Iraq's Special Forces, Lieutenant General Talib Shaghati, told reporters at a command centre near the frontline that troops had surrounded Bartella and entered the centre of the village. Two soldiers were hurt and none killed, and they had killed at least 15 militants, he said.

"After Bartella is Mosul, God willing."

A cloud of black smoke wreathed some frontline villages, probably caused by oil fires, a tactic the militants use to escape air surveillance.

Prime Minister Haidar al-Abadi, addressing anti-Islamic state coalition allies meeting in Paris by a video link, said: "The forces are pushing towards the town more quickly than we thought and more quickly than we had programmed."

A U.S.-led coalition that includes France, Italy, Britain, Canada and other Western nations is providing air and ground support to the forces that are closing in on the city in an operation that began on Monday.

Mosul is the last big stronghold held by Islamic State in Iraq and around five times the size of any other city the group has held. The push to capture it is expected to become the biggest battle fought in Iraq since the 2003 U.S.-led invasion.

The United Nations says Mosul could require the biggest humanitarian relief operation in the world, with worst-case scenario forecasts of up to a million people being uprooted by the battle.

Some 1.5 million residents are still believed to be inside the city, and Islamic State fighters have a history of using civilians as human shields.

French Foreign Minister Jean-Marc Ayrault said controls were being put in place to check jihadists were not trying to insert themselves among those fleeing Mosul.

BOMBS, BUNKERS AND TUNNELS

On the northern front, Kurdish forces known as peshmerga shot down a small drone that had flown over from the Islamic State lines. It was not clear if the drone, 1 to 2 metres (yards) wide, was carrying explosives or being used for reconnaissance.

"There have been times when they dropped explosives," said Halgurd Hasan, one of the Kurdish fighters deployed in a position overlooking the plain north of Mosul.

Ali Awni, a Kurdish officer, kept a handheld radio receiver open on a frequency used by Islamic State. "They are giving targets for their mortars," he said.

"Liberating Mosul is important for the security of Kurdistan," Awni added. "We will have to fight them in the mind as well, to defeat their ideology."

So far, advancing Kurdish troops have moved through villages outside the city, finding abandoned houses rigged with explosives and underground bunkers. In some cases, Islamic State fighters appear to have fled without putting up a fight.

"We did not face resistance from Daesh. They are retreating to Mosul and to Syria. They gave no resistance," peshmerga soldier Ahmed Midhat Abdullah told Reuters in the village of Nawaran, north of Mosul, where a Kurdish column of armoured vehicles was advancing in the dusty desert terrain.

Daesh is an Arabic acronym for Islamic State, also known as ISIS or ISIL, which swept into Mosul and other parts of northern Iraq in 2014 and has used extreme violence to administer a self-proclaimed caliphate there and in parts of neighbouring Syria.

"The objectives are to clear a number of nearby villages and secure control of strategic areas to further restrict ISIL's movements," the Kurdish general military command said in a statement announcing the launch of Thursday's operations.

SHI'ITE MILITIA

The area around Mosul is one of the most ethnically and religiously diverse parts of Iraq, and Western countries backing the assault are concerned that communities feel safe as the government forces advance, to avoid revenge attacks or ethnic and sectarian bloodletting as fighters are driven out.

Western allies have sought to limit the role of Shi'ite militia fighters known as the Popular Mobilization Forces, which human rights groups say have carried out killings and kidnappings of Sunnis in other areas freed from Islamic State.

After the Paris meeting, Iraq's foreign minister offered reassurance: "In answer to those who criticise the PMF for behaving badly, this is not true ... They are part of the Iraqi forces and will be disbanded afterwards."

Abadi said the Mosul advance demonstrated that Iraqis from all groups could fight in common cause, noting that it was the first time in 25 years that troops from the Baghdad government had entered territory controlled by the Kurdish region to fight alongside the peshmerga.

"Our war today in Mosul is an Iraqi war conducted by Iraqis for Iraqis and for the defence of Iraq's territory," he said. "Full Iraqi unity is shining through and more than ever showing the unity to vanquish terrorism."

Islamic State published a video showing masked fighters walking in single file up a street at night under the cover of trees, while an unidentified man, apparently their commander, pledged to defeat the United States in Iraq.

U.S. President Barack Obama hopes to bolster his legacy by seizing back as much territory as he can from Islamic State before he leaves office in January.

Islamic State "will be defeated in Mosul", Obama said on Tuesday, expecting the fight to be difficult.

Iraqi officials and residents of Mosul say Islamic State is preventing people from leaving the city, in effect using them as shields to complicate air strikes and the ground progress of attacking forces.