KIRKUK - The Iraqi army is deploying thousands of soldiers to a northern base in preparation for operations to retake the Islamic State group's hub of Mosul, officials said on Monday.

IS seized Mosul in June 2014 and except for air strikes has held Iraq's second city largely unopposed for more than 18 months as the country's security forces battled the militants in other areas.

"Units from the Iraqi army have begun arriving to a military base near the Makhmur district to start launching initial military operations toward Mosul," a staff brigadier general told AFP on condition of anonymity. Makhmur lies around 70 km southeast of Mosul. "There are three brigades located in that base now," and their number will eventually reach 4,500 soldiers, said the officer, who is part of the security command tasked with retaking Nineveh province, of which Mosul is the capital. Iraqi forces recaptured Ramadi, the capital of Anbar province, west of Baghdad, in December in a major blow to the militants.

But Mosul is expected to be the most difficult battle of Iraq's war against IS due to its size, the presence of large numbers of civilians in the city, and the long period the militants have had to prepare defences. It is the largest city held by IS, and along with Raqa in Syria is one of the two main hubs for the militants' operations.

The initial aim is to sever IS supply lines between Mosul and areas farther south including Hawijah in Kirkuk province and areas near Baiji in Salaheddin, the army officer said.

Iraqi forces have worked slowly and deliberately to isolate other IS-held cities such as Tikrit and Ramadi before launching assaults to retake them, and cutting off Mosul will likely take longer than either.

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.

A Kurdish official said that most of the soldiers deployed to Makhmur, which is located within territory controlled by Iraqi Kurdistan, are ethnic Kurds from the Iraqi army. "These forces came with the approval of the presidency and government of the Kurdistan region of Iraq," said Halgurd Hekmat, a spokesman from the ministry responsible for the autonomous region's peshmerga forces.

The base will be for both soldiers and aircraft, Hekmat said.

Shakhwan Abdullah, a Kurdish lawmaker on the federal parliament's security and defence committee, confirmed that Baghdad and Iraqi Kurdistan had coordinated on the deployment. "Establishing this base cost around $10 million," said Abdullah, adding that the base would be turned over to Kurdish peshmerga forces after the recapture of Mosul.

Defence Minister Khaled al-Obeidi visited the Makhmur base on Saturday, ordering that all measures be taken to speed up the completion of the site, the ministry said.

Multiple Iraqi divisions collapsed during the early days of IS's 2014 offensive, abandoning thousands of vehicles as well as weapons and other equipment to the militants.

IS ultimately overran around a third of the country, but Iraqi forces have since regained significant areas from the militants, with the help of US-led air strikes and training.

Meanwhile, doctors and traffic police protested Monday in the cash-strapped Kurdish region of northern Iraq, as discontent mounted over unpaid salaries and wage cuts in the government sector.

In and around the city of Sulaimaniyah, members of the traffic police staged a sit-in and went on strike because their salaries have not been paid for months.

"Traffic police officers and members have begun a strike in several areas of the province (of the same name) to demand a solution to the salaries issue," local traffic police directorate spokesman, Karwan Mohammed Siddiq, told AFP.

Teachers led the way two weeks ago with protests over months of unpaid wages, also in Sulaimaniyah. The autonomous Kurdish region has been hit hard by the slump in oil prices and its de facto president, Massud Barzani, announced earlier this month that all government employees except security personnel would only receive partial salaries.

In Iraqi Kurdistan's capital Arbil, where the ruling party is dominant, doctors sent the regional government a petition Monday demanding they be treated like the security forces and paid full salaries.

They argued they were an integral part of the war effort led by the peshmerga forces against the Islamic State militant group because they helped treat fighters and their families. The doctors threatened to "hold a general strike if the government does not respond to these demands by February 14".