As troops advance, up to 400,000 civilians are at risk of being displaced by the offensive. Residents of western Mosul have been facing food and fuel shortages, according to the UN humanitarian coordinator for Iraq.
Iraqi forces have launched an operation to retake the western half of Mosul from the Daesh militant group.
Iraqi Prime Minister Haider al-Abadi announced the start of the operation early Sunday morning on state television. "We call on our brave troops to start the push to liberate the people of Mosul from Daesh oppression and terrorism forever," al-Abadi said using the Arabic acronym for IS.
Iraqi forces took control of eastern Mosul last month, but the west remains in the hands of Daesh. The militants are now under siege in western Mosul, along with an estimated 650,000 civilians, after US-backed forces surrounding the city forced them from the east in the first phase of an offensive that concluded last month.
In the current phase of the operation, Iraqi federal police units are leading a northward charge on Mosul districts that lie west of the Tigris river, aiming to capture Mosul airport, just south of the city, according to statements from the armed forces joint command.
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Iraqi forces captured several villages and a local power distribution station in the first hours, and killed several militants including snipers, the statements said.
The police are advancing up the Tigris river valley towards the airport. The Rapid Response, an elite Interior Ministry unit, cut across more open terrain to the southwest.
"Mosul would be a tough fight for any army in the world," the commander of the US-led coalition forces, Lieutenant General Stephen Townsend, said in a statement.
Two militants blew themselves up in eastern Mosul on Sunday, killing three soldiers and two civilians, and wounding a dozen people, security sources said.
"We reiterate as we said before, the importance of taking care of people and dealing with them in a humanitarian way and providing all the necessary requirements for the displaced people as you did before, you brave fighters," the prime minister had said on television.
Iraqi planes dropped millions of leaflets on western Mosul warning residents that the offensive was imminent, the Defence Ministry said on Saturday.
Daesh in Iraq and Syria
Mosul, Iraq's second-largest city, is roughly split in half by the Tigris River. The battle for Mosul's western half is expected to be prolonged and difficult, due to denser population and older, narrower streets.
The offensive to dislodge Daesh from Mosul, its last major city stronghold in Iraq, started in October. In 2014, the hardline group declared a self-styled caliphate that also spans parts of Syria.
Mosul is the largest city it captured in both countries and its de facto capital in Iraq. Raqqa is its capital in Syria.
The defeat of the group in Mosul would effectively end the Iraqi half of the "caliphate."