Iraqi forces have launched an operation to retake the city of Mosul from the Islamic State, a city whose capture by the group two years ago left the country on the brink of collapse.
At least five times bigger than any other IS-controlled city, Mosul's liberation would effectively eliminate the group's self-declared caliphate from Iraqi territory.
The offensive will be the country's biggest Iraqi military operation since the withdrawal of American troops in 2011, with some 30,000 troops from Iraq's armed forces, Kurdish Peshmerga militia, and Sunni tribal factions expected to take part, backed by a 60-nation US-led coalition.
Estimates put the number of Islamic State militants in the city between 4,000 to 8,000.
Iraqi forces could be seen readying weapons and ammunitions as columns of armored vehicles headed towards Mosul from the town of Al-Shura, some 45 kilometers (30 miles) south of the city, an AFP photographer said.
As the assault began, federal forces moved from their main staging base of Qayyarah, south of Mosul, while peshmerga forces from the autonomous Kurdish region simultaneously advanced from the east.
Announcing the start of the operation in a televised address early Monday, Iraqi Prime Minister Haider al-Abadi said that only the Iraqi army and police would enter the city, as he sought to quell concerns that the operation could lead to sectarian conflict among the coalition of myriad and sometimes rival Iraqi forces.
"Today I declare the start of these victorious operations to free you from the violence and terrorism of Daesh," Abadi said, using the Arabic name for the jihadist group.
"The forces that lead the liberation operation are the brave Iraqi army with the police forces," Abadi said. "They will enter the city and no one else," he added, urging Mosul's civilian population to cooperate with the government's forces.
US Defense Secretary Ash Carter said in a statement the offensive was "a decisive moment in the campaign to deliver [IS] a lasting defeat."
- UN 'extremely concerned' -
The beginning of the assault also saw aid groups voice fears for up to 1.5 million civilians remaining in the city, with IS expected to use them as human shields.
The jihadists have had months to prepare and will seek to use hit-and-run tactics, ambushes, snipers, bombs, berms and trenches to slow down and bleed Iraqi forces.
"I am extremely concerned for the safety of up to 1.5 million people living in Mosul who may be impacted by military operations to retake the city from [IS]," UN deputy Secretary General for Humanitarian Affairs and Emergency Relief Stephen O'Brien said, warning that "families are at extreme risk of being caught in cross-fire or targeted by snipers."
UK-based charity Save the Children said in a statement that around 600,000 children living in Mosul could be caught in the crossfire.
The United Nations warned last week that the operation could lead to an unprecedented humanitarian crisis, threatening to displace as many as one million people from the city where medical and food shortages were already causing concern.
The Iraqi military dropped leaflets over the city before dawn on Sunday announcing the impending battle, Reuters cited a military statement as saying.
Aircraft dropped "tens of thousands of newspapers and magazines on the center 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.
The leaflets said the offensive "will not target civilians" and to remain at home, reflecting concerns that a mass civilian evacuation could complicate the operation.
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.
Russian President Vladimir Putin said Sunday that he hoped the US and its allies would work to avoid civilian casualties in the offensive.
- Last stand -
The push for Mosul comes 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.
IS once controlled more than a third of Iraq's territory but its self-proclaimed "state" has been shrinking steadily for more than a year.
It lost its emblematic bastion Fallujah in June, the Anbar provincial capital Ramadi months earlier and on Sunday Syrian rebels retook the town of Dabiq, which held special ideological significance for the group.
A clear victory in Mosul would go some way towards restoring the confidence and credibility of the Iraqi security forces, which collapsed when IS attacked two years ago, abandoning huge amounts of US military equipment to the group in the process.
But even the recapture of Mosul will not mark the end of the war against IS, which is likely to increasingly turn to insurgent tactics such as bombings as it loses more ground.
The jihadists' defeat in Mosul appears to be only a matter of time and some military sources suggest an exit route for fleeing jihadists could be left open in a bid to minimise the fighting and its potential impact on the city's population and infrastructure.
The battleground is vast and the area's ethnic and religious diversity makes it politically complex.
IS has attempted to erase the ancient city's history, forcing Christians and other minorities to flee and destroying priceless heritage.
(Staff with agencies)