Iraqi forces say they have made progress on day one of their offensive to drive out Islamic State from Iraq's second-biggest city in the country's north.
They say they have taken several villages in their push towards Mosul airport.
Iraq's Ministry of Defence has released footage of its air strikes on IS targets in the western flank.
The commander of the army aviation, Lieutenant General Hamid al-Maliki, says it has been a successful start.
"Our helicopters took off in the early hours of the start of the battle to back up the troops of al-Hashd al-Shaabi Popular Mobilisation Forces and carried out 22 sorties. They made marvellous results. They were able to kill 93 Islamic State militants and destroyed 18 military and armoured vehicles which were being used by Islamic State."
Iraqi forces took control of eastern Mosul last month, but the city's west remains in the hands of the militants.
Commanders expect the battle to be more difficult than in the east because tanks and armoured vehicles cannot pass through its narrow alleyways.
Residents say the militants have a network of passageways and tunnels to help them hide and fight among civilians, vanish after hit-and-run operations and track government troops.
Western Mosul contains the old city centre, with its ancient souks and Grand Mosque and most government administrative buildings.
Iraqi prime minister Haider al-Abadi has set out the mission's priorities.
"I call on our brave troops to start the push to liberate the rest of the city and to liberate people from the oppression and terrorism of Daesh. Our main mission is to liberate the people before liberating the land. You have done a great job by liberating other areas, by taking care of people -- including the displaced people -- and by providing services for them. We reiterate, as we said before, the importance of taking care of the 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."
United States defence secretary Jim Mattis has refused to be drawn on specifics of the operation.
He says only that US troops are helping the Iraqi forces against IS, also known as ISIS or ISIL.
"The US forces continue in the same role that they were in in east Mosul, and the coalition forces are in support of this operation, and we will continue, as you know, with the accelerated effort to destroy ISIS."
Around 650,000 people are believed to be trapped in western Mosul.
The United Nations estimates up to 400,000 could be displaced by the offensive as residents suffer food and fuel shortages and markets are closed.
The Iraqi Defence Ministry says Iraqi planes dropped millions of leaflets on western Mosul warning residents the battle to dislodge IS was imminent.
The leaflets told IS members to surrender or face what they called "a fatal end."
But the group is not giving up without a fight.
There are reports IS unleashed two attacks on government-controlled eastern Mosul in the hours before the offensive began.
The owner of a restaurant frequented by security forces told the BBC he is too scared to be identified.
"The situation is very bad. We are opening only two hours a day. An attack could happen any time. ISIL threatened to carry out more attacks when civilians and security personnel gather around restaurants and cafes. That's why we are so afraid."
