Iraqi forces advanced Monday after launching an offensive aimed at retaking Mosul and dealing a death blow to the Islamic State group's "caliphate" in the city where it was declared two years ago.

The start of the long-awaited assault raised deep concerns for hundreds of thousands of civilians trapped in Iraq's second-largest city, with aid groups warning of a massive humanitarian crisis.

Some 30,000 federal forces are leading the offensive, backed by air and ground support from a 60-nation US-led coalition, in what is expected to be a long and difficult assault on IS's last major Iraqi stronghold.

Iraqi forces could be seen readying weapons and ammunitions as columns of armoured vehicles headed towards Mosul from the town of Al-Shura, some 45 kilometres (30 miles) south of the city.

The Pentagon described the operation as a "decisive moment" in the fight against IS but the US-led coalition's top commander warned it could last weeks or more.

Iraqi Prime Minister Haider al-Abadi announced the beginning of the assault in a televised address in the early hours of Monday.

"Today I declare the start of these victorious operations to free you from the violence and terrorism of Daesh (IS)," Abadi said.

IS seized control of large parts of Iraq and neighbouring Syria in mid-2014, declaring a cross-border "caliphate" and committing widespread atrocities.

- Toughest battle yet -

The jihadists have suffered a string of territorial defeats this year in both countries, and the retaking of Mosul would all but end the group's presence in Iraq as a land-holding force.

Iraqi forces have been closing in on Mosul in recent weeks but the battle launched on Monday could be the toughest yet in the fight against IS.

IS forces are vastly outnumbered, with the US military estimating that 3,000 to 4,500 jihadists are in the city and surrounding area.

But they have had months to prepare and will seek to use hit-and-run tactics, ambushes, snipers, bombs, berms and trenches to slow down and bleed Iraqi forces.

In a statement released online after the beginning of the assault, IS claimed it launched a series of deadly suicide car bomb attacks against Iraqi forces.

As the assault began, federal forces moved from their main staging base of Qayyarah, south of Mosul, while peshmerga forces from the autonomous Kurdish region advanced from the east.

Around 4,000 Kurdish peshmerga took part in a push to reclaim villages once inhabited by members of the Christian and Kakai minorities, a statement said.

Several villages were promptly recaptured and peshmerga forces had moved to the edges of Qaraqosh and Bartalla, two Christian towns seized by IS in August 2014, commanders said.

At a staging area near the village of Khazir east of Mosul, Kurdish forces could be seen firing artillery and rockets as smoke rose from houses in the distance.

The United Nations and other aid organisations warned that a huge number of civilians were in immediate danger.

"I am extremely concerned for the safety of up to 1.5 million people," the UN deputy secretary general for humanitarian affairs and emergency relief, Stephen O'Brien, said.

- Exodus fears -

The UN's humanitarian coordinator in Iraq, Lise Grande, told reporters that an exodus could begin within a week.

"It's a trickle right now," she said.

But "our expectation, based on what the military has briefed us (on), is that if we start to see significant population movements, it's likely to be within five to six days."

The Norwegian Refugee Council said safe exits from Mosul must be provided or civilians "will be faced with the bleakest of choices: stay behind and risk their lives under attack, or risk their lives trying to flee."

Iraqi forces advancing on multiple fronts are some distance from Mosul and are expected to eventually take up positions on the edge of the city and lay siege before breaching its boundaries and directly engaging die-hard jihadists.

"This operation to regain control of Iraq's second-largest city will likely continue for weeks, possibly longer," warned Lieutenant General Stephen Townsend, commander of the US-led coalition.

Most of the coalition's support has come in the shape of air strikes and training but US, French and British special forces are also on the ground to advise local forces in battle.

"This is a decisive moment in the campaign to deliver ISIL a lasting defeat," US Secretary of Defence Ash Carter said, using an alternative acronym for the jihadist group.

President Recep Tayyip Erdogan on Monday indicated that Turkey would also play a role in the offensive despite concerns raised by Baghdad over Ankara's involvement.

"We will be in the operation and we will be at the table," Erdogan said in a televised speech. "It is out of the question that we are not involved."

- Last stand -

As the biggest Iraqi military operation since the 2011 withdrawal of US troops unfolded, a Turkish delegation arrived in Baghdad for talks and coalition defence ministers agreed to meet in Paris on October 25 to take stock.

IS once controlled more than a third of Iraq's territory but its self-proclaimed "state" has been shrinking steadily.

It lost its emblematic bastion Fallujah in June, the Anbar provincial capital Ramadi months earlier and on Sunday Syrian rebels retook the town of Dabiq, which held special ideological significance for the group.

If Mosul falls to Iraqi forces, only Raqa in Syria would remain as the last major city in either country under IS control.

But even the recapture of Mosul will not mark the end of the war against IS, which is likely to increasingly turn to insurgent tactics as it loses more ground.

Just hours after the offensive was launched, IS claimed responsibility for a suicide car bombing targeting an Iraqi army checkpoint south of Baghdad that killed at least 10 people.

The jihadists have claimed a string of deadly bomb attacks in Baghdad in recent days.