Iraqi government forces have launched an offensive to liberate the western part of the city of Mosul from so-called Islamic State.
Military sources say at least two villages have been recaptured from jihadists.
Smoke rose over villages as hundreds of military vehicles, backed by air power, rolled across the desert towards the jihadists' positions on Sunday.
Iraqi Prime Minister Haider al-Abadi formally announced the offensive.
Government forces retook the eastern side of the city, the last major IS stronghold in Iraq, last month.
The western side, with its narrow, winding streets, may prove a bigger challenge.
The United Nations (UN) has voiced concern about civilians trapped there, amid reports that they could number up to 650,000.
Leaflets warning residents of an imminent offensive were earlier dropped over the west of the city.
"We announce the start of a new phase in the operation, we are coming to Nineveh to liberate the western side of Mosul," Mr Abadi said in a televised speech, referring to the province of which Mosul is the capital.
"Our forces are beginning the liberation of the citizens from the terror of Daesh [IS]," he added, quoted by AFP news agency.
Iraqi forces have all but surrounded the western part of Mosul, while the US-led coalition has carried out air strikes on IS targets.
Winning the city back has never been just about driving IS out of its stronghold, BBC reporter Sebastian Usher said.
Minimising civilian casualties and avoiding reprisals was key to restoring people's trust in the Iraqi state, he said.
The UN humanitarian co-ordinator for Iraq, Lise Grande, said "all of the parties to the conflict do absolutely everything they can to ensure that civilians survive the battle, and that they live".
"Absolutely nothing is more important going into the campaign to retake western Mosul," she said.
The offensive on the eastern part of the city was launched on 17 October, more than two years after jihadists overran Mosul before seizing control of much of northern and western Iraq.
Government forces managed to secure the eastern part last month after fierce fighting.
Experts warn that western Mosul, although slightly smaller than the east, is more densely populated and includes districts seen as pro-IS.
The UN said in late January almost half of all the casualties in Mosul were civilians.
At least 1,096 have been killed and 694 injured across Nineveh province since the start of October.