Iraqi forces have launched operations to retake Mosul, a city whose capture by the Islamic State group two years ago left the country on the brink of collapse, the prime minister said Monday.

The northern city was where IS supremo Abu Bakr al-Baghdadi publicly proclaimed a "caliphate" straddling Iraq and Syria in June 2014.

With the support of Iran and a US-led coalition, Iraqi forces have since regained much of the ground lost to IS and Mosul is the extremist group's last major stronghold in Iraq.

"Today I declare the start of these victorious operations to free you from the violence and terrorism of Daesh (IS)," Prime Minister Haider al-Abadi said in a televised address.

The commander-in-chief of Iraq's armed forces did not divulge details of the latest movements on the ground but forces have recently been tightening the noose around Mosul.

A coalition of myriad and sometimes rival Iraqi forces -- including the US-led coalition, Kurdish Peshmerga and Iraqi government forces -- will have to fight their way through IS defences to reach the city, in some cases over distances of dozens of kilometres.

Then they will likely seek to surround the city before launching an assault, marking the start of deadly street fighting with die-hard jihadists.

Abadi vowed that only government forces would enter Mosul, a Sunni-majority city that IS seized with relative ease partly because of local resentment towards the Shiite-dominated security forces.

Shiite militia groups have been accused of serious abuses against Sunni civilians in the course of operations to reconquer territory from IS.

"The force leading liberation operations is the brave Iraqi army with the national police and they are the ones that will enter Mosul, not others," Abadi said.

Baghdad is also keen to check the influence of the autonomous Kurdish region, which significantly expanded the territory it controls on the back of the 2014 jihadist assault.

- Final push -

The Hashed al-Shaabi paramilitary organisation, which is dominated by Tehran-backed militia groups, has made clear it wants to take part in the Mosul operation.

Iraqi Kurdish peshmerga forces have been moving in from the eastern side of the city while a US-led coalition is also providing support in the air and on the ground.

Turkey, which borders northern Iraq, has also offered to join the offensive on Mosul.

The start of the much-delayed offensive on Mosul risks sparking a humanitarian crisis, with the United Nations warning that up to one million people may be displaced by the fighting as winter sets in.

The jihadists are not thought to number more than 5,000 and will be vastly outnumbered but the battleground is vast and the area's ethnic and religious diversity makes it politically complex.

Abadi's announcement came a day after rebel forces in the Syrian half of IS' shrinking "caliphate" retook Dabiq, a town which holds crucial ideological significance for the group and where it had promised an apocalyptic battle.

Iraqi forces recently retook Qayyarah, an area some 60 kilometres (35 miles) south of Mosul, setting the stage for the final push on IS's northern bastion.

The force that has led operations to recapture other IS strongholds in Iraq, such as Fallujah in June or Ramadi earlier this year, is the elite counter-terrorism service.

Thousands of other army and police forces have also been converging on Mosul in recent weeks.

Abadi has promised that Iraq would be rid of the most violent organisation in modern jihad by year's end.

But even the recapture of Mosul will not mark the end of the war against IS, which still holds other territory in Iraq and is likely to turn increasingly to insurgent tactics such as bombings and hit-and-run attacks as it loses more ground.

Forces operating in Iraq's Mosul theatre
Baghdad (AFP) Oct 17, 2016 - A wide array of Iraqi and international forces are involved in the fight to retake Mosul from the Islamic State jihadist group, which overran the country's second city in 2014.

Prime Minister Haider al-Abadi announced early Monday that operations to retake Mosul had started.

These are the main forces that may operate in the Mosul theatre, though not all will play a role in the fighting inside the city:

- Islamic State group -

Heavily-armed jihadists who have had years to prepare their defences in Mosul, which IS seized before sweeping through cities and towns to the south in 2014.

Iraqi forces have since regained significant ground, and Mosul is the last city the jihadists hold in the country.

- Counter-terrorism service -

The most elite forces in the country who have spearheaded most key battles against IS. But constant reliance on these troops over the past two years has taken a toll.

- Army -

The Iraqi army has begun playing a more successful role in operations against the jihadists since it was revitalised by US-led training following several of its divisions collapsing during the IS offensive in the north two years ago.

- Police -

Includes special forces units, paramilitary federal police and local policemen. Many Iraqi police forces have played roles more akin to those of soldiers in the war against IS.

- US-led coalition -

A US-led international alliance is carrying out air strikes against IS in Iraq and Syria, and providing training, arms and equipment to forces opposing the jihadists.

There are more than 7,500 coalition military personnel deployed in Iraq, over half of them from the United States.

Most are in advisory or training roles, but special forces soldiers who have fought the jihadists on the ground have also been deployed and coalition forces near Mosul have also targeted IS with artillery.

- Peshmerga -

Security forces of Iraq's autonomous Kurdish region who nominally answer to the federal government but in practice operate independently, battling IS along a long front in the country's north.

- Hashed al-Shaabi -

An umbrella organisation created in 2014, which includes a dizzying array of paramilitary forces who vary widely in skill and in the degree to which they are actually under government control.

The main groups in the Hashed al-Shaabi (Popular Mobilisation) are Iranian-backed Shiite militias including Ketaeb Hezbollah, Asaib Ahl al-Haq and Badr.

The Hashed has played a major role in stopping IS's advance as well as regaining ground from the jihadists, but forces within it have also carried out abuses including summary executions and kidnappings.

- Iranian advisers -

Iranian forces have provided advice and other assistance including funding for various militias fighting IS in Iraq.

Qassem Soleimani, the commander of Iran's Revolutionary Guards foreign operations wing, has been repeatedly pictured in Iraq during the war.

- Turkish troops -

Deployed at a base near Mosul from which they have carried out artillery strikes against IS, Turkish troops are also present inside Iraq's autonomous Kurdish region.

The federal government has demanded their withdrawal, with the prime minister vowing that they will not take part in the operation to recapture Mosul, but Turkey has declined to do so.