BAGHDAD (AFP) - The Iraqi army said Thursday (March 24) 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 in Iraq and Syria (ISIS) 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 ISIS, 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 ISIS 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 ISIS, including by last month retaking Anbar provincial capital Ramadi.

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

But Mosul - which along with Raqa in Syria is one of the militants' 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 militants 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 ISIS, 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.

Thousands of troops were deployed in February to a base in Makhmur, some 70km south-east 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 ISIS-used bridge section and denied ISIS 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 ISIS 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 ISIS.

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

ISIS 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, ISIS 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 ISIS in Iraq in August of 2014 and has killed thousands of the militants.

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.