The Iraqi army said Thursday its troops and allied militia had launched what is expected to be a long and difficult offensive to retake the second city of Mosul, the Islamic State group's main hub in Iraq.

The army and the Popular Mobilisation paramilitary force "have begun the first phase of conquest operations" in the northern Nineveh province, of which Mosul is the capital, Iraq's joint operations command said in a statement.

It said four villages had been taken between the town of Qayyarah, which is still held by IS, and Makhmur, where US-backed Iraqi forces have been massing in recent weeks.

The army did not say how long this phase of the operation was expected to take and Iraqi forces still look far from being in a position to take the city itself.

The joint operations command is coordinating the battle by Iraqi security forces to retake the large parts of the country seized by IS during a lightning offensive in 2014.

It includes representatives from the US-led coalition that has provided air support, training and military advisers for the Iraqi army in its fightback.

Iraqi forces have scored important recent gains against IS, including by last month retaking Anbar provincial capital Ramadi.

The latest announcement comes as pro-government forces in Syria closed in on IS in the ancient city of Palmyra, which the jihadists seized around the same time as Ramadi last year.

But Mosul -- which along with Raqa in Syria is one of the jihadists' two main hubs -- would be a major prize.

Experts have warned that any battle to retake the city will be difficult, given the significant number of jihadists and civilians in the city and the time IS has had to prepare defences.

Lieutenant General Sean MacFarland, the commander of the US-led operation against IS, has said that Iraqi generals do not think they will be able to recapture Mosul until the end of 2016 or early 2017 at the earliest.

As they have done in battles to retake cities like Ramadi and Tikrit, Iraqi forces are expected to work slowly and deliberately to cut off supply lines to Mosul before launching an assault on the city.

The International Committee of the Red Cross said it distributed aid on Thursday to more than 12,000 displaced people in west Ramadi who fled fighting in the nearby city of Hit.

- Long way to Mosul -

Thousands of troops were deployed in February to a base in Makhmur, some 70 kilometres (45 miles) southeast of Mosul, in preparation for the offensive.

The US-led coalition said it carried out three strikes in the Qayyarah area on Wednesday.

"Near Qayyarah, three strikes struck an ISIL (IS) communication facility and destroyed an ISIL-used bridge section and denied ISIL access to terrain," it said in a statement.

It also launched eight strikes in the broader Mosul region.

Peshmerga fighters of Iraq's autonomous Kurdish region have also been heavily involved in the campaign against IS in northern Iraq.

The peshmerga deputy commander for the sector, Araz Mirkhan, confirmed to AFP on Thursday that the offensive had started.

"Iraq forces in Makhmur have begun their advance towards Qayyarah to the south of Mosul," he said, referring to the town on the Tigris River to the west of Makhmur.

"The advance has allowed us to liberate four or five villages from the Daesh terrorists," he added, using an Arabic acronym for IS.

Iraqi forces collapsed in the face of the 2014 IS advance and the jihadist group ultimately overran around a third of the country.

IS has declared an Islamic "caliphate" in areas under its control in Iraq and in neighbouring Syria, where it has also seized significant territory.

Imposing its extremist interpretation of Islamic law, IS has committed widespread atrocities in areas under its control and launched a wave of attacks against the West, including this week's bombings in Brussels that killed 31 people.

The US-led coalition of Western and Arab nations launched air strikes against IS in Iraq in August of 2014 and has killed thousands of the jihadists.

The top US envoy to the coalition, Brett McGurk has said several times since the start of the year that the operation to liberate Mosul was already underway.

"Mosul has kind of already started, but you've got to think of it of a rolling kind of campaign to isolate and squeeze," he said.

Mosul: Iraq's second city
Baghdad (AFP) March 24, 2016 - Mosul was an Iraqi demographic mosaic of Arabs, Kurds, Turkmens and Christians that was gutted after Islamic State fighters swept in on June 10, 2014.

Iraq's second-largest city, with a population now estimated at just over one million, mostly Sunni Arabs, Mosul is crucial to IS, which declared its Islamic "caliphate" there.

The city controls strategic trade routes in northern Iraq, notably a key highway to the border with Syria and its second city of Aleppo.

Other routes lead to Turkey and to Baghdad, while a smaller road heading west through the Sinjar mountains to Syria has been used for centuries as a smuggling route.

Food and fabrics have long been traded in Mosul, which is also known for producing the fine cotton fabric called muslin.

Capital of the oil-rich Nineveh province, Mosul lies around 350 kilometres (220 miles) north of Baghdad, and 50 kilometres (30 miles) south of Iraq's biggest dam, now controlled by Kurdish peshmerga fighters.

The city's historic centre is dotted with church spires and it was home to an estimated 35,000 Christians when IS arrived and ordered them to convert, pay a special tax, or leave. Almost all fled.

Mosul was conquered by Arabs in 641 and reached its cultural peak in the 12th century before falling to Mongols in 1262, and then to Persians and Ottomans.

The city became part of Iraq when the country was created out of the ashes of the Ottoman Empire in the 1920s.

Nineveh has always been a border region, keenly contested by its rival communities and their more powerful supporters in neighbouring states.

Under the ousted Sunni-dominated regime of Saddam Hussein, Mosul saw a huge influx of immigrants that boosted its population to around two million.

Disproportionate numbers were from Arab areas of the countryside, changing its demographic make-up, so that Arabs now outnumber Kurds.

For the most part the two ethnic groups live on opposite sides of the Tigris river that runs through the city.

Mosul proved a bastion of Saddam's most dedicated supporters who became a foundation of IS.

After chasing Iraqi troops out, IS set about destroying Mosul's cultural heritage, burning thousands of rare books and manuscripts in the city's vast library and smashing priceless statues.

In July 2014, IS fighters rigged the Nabi Yunus shrine -- revered by both Muslims and Christians as the tomb of Prophet Jonah -- with explosives and blew it up.

The population is now subject to a strict interpretation of Islamic law, and IS fighters have reportedly planted more munitions in many parts of the city in anticipation of the Iraqi counter offensive.