Ammon News - AMMONNEWS - Iraqi security forces are preparing for a major offensive in northern Iraq to retake Mosul, the last ISIL-held city in the country.

Ahead of the planned operation, Iraqi aircraft dropped "tens of thousands" of leaflets early on Sunday, some bearing safety instructions for Mosul residents, the military said.

The bid to retake ISIL's de facto capital in Iraq comes after the military, backed by armed tribes and militias, regained much of the territory the fighters seized in 2014 and 2015.

The launch of the operation is expected to be announced soon, but it will mark only the start of a battle that is likely to be the most difficult and complex yet in the war against ISIL.

Al Jazeera's Zeina Khodr, reporting from Khazir, just east of Mosul, said that artillery bombardment targeted the eastern sections of Mosul earlier on Sunday but the offensive had still not begun in full swing.

"We are waiting for a formal announcement from the Iraqi Prime Minister Haider al-Abadi," Khodr said.

"There's been preparations for quite some time now for this battle, with troops amassing around the city where an estimated one million people still live there under the control of ISIL."

Aircraft dropped "tens of thousands of newspapers and magazines on the centre of the city of Mosul carrying important news ... to inform them of updates and facts and victories", said Iraq's Joint Operations Command, which distributed images of some of the leaflets.

One image showed a leaflet containing safety instructions for Mosul residents, urging them to tape over windows to prevent the glass from shattering, to avoid the sites of air strikes for at least an hour after a place is bombed, and saying they should not drive if possible.

Iraq has dropped leaflets over Mosul before, and has also done so as part of operations to retake other cities seized by ISIL in 2014 and 2015.

"According to Iraqi officials, the battle plans have been drawn up to make sure that civilians are protected and not caught in the crossfire," Khodr said.

"ISIL has been fortifying its positions for some time now; we understand that they have dug a trench around the city and we also understand that they plan to light fire once the offensive begins because the smoke will obscure the vision of the aircraft."

According to UN estimates, up to one million people could be displaced from Mosul during the operation, exacerbating the humanitarian situation in the country.

The conflict in Iraq has forced 3.3 million people to flee their homes, with most of them living in camps or informal settlements.

The preparations came as a suicide bombing targeting Shia Muslims killed at least four people in Baghdad on Sunday, officials said, a day after the deadliest attack to hit the Iraqi capital in months.

The bombing in central Baghdad, which targeted a tent where Shias distribute food as part of annual religious commemorations, also wounded at least 12 people, officials said.

There was no immediate claim of responsibility for the attack, but the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group frequently carries out suicide bombings targeting Shias, whom it considers heretics.

The blast came a day after an ISIL-claimed suicide bombing at a funeral killed at least 34 people - the deadliest attack in Baghdad since another ISIL suicide bombing left more than 300 dead in early July .

*Agencies