Residents have said IS supporters inside the city, which has been controlled by the group for more than two years, have been increasingly violent since the start of the military operation.

One man was shot in the head in front of his family and neighbours on Sunday for having a mobile phone SIM card, witnesses told the Associated Press news agency.

A few days earlier, five other men were killed by a firing squad in a public square.

"The situation in Mosul is terrifying," a merchant said. He added that he had stocked up 40 days of food, water and cooking gas.

US defence officials have accused IS of stopping 1.5 million civilians leaving the city so it can use them as human shields.

"They are being held there against their will," Pentagon spokesman Navy Captain Jeff Davis. "We have not seen any change in the last day of people leaving or fleeing."

United Nations spokesman Stephane Dujarric said an estimated 200,000 people were expected to be displaced in the first weeks of the offensive, growing to as many as one million under a worst-case scenario.

People flee their homes during clashes between Iraqi security forces and IS fighters | Photo: PA Images

Progress towards Iraq's second largest city slowed on Tuesday as Iraqi and Kurdish forces were attacked by suicide bombers and hit by roadside bombs.

Brigadier Yahia Rassoul, a spokesman for the Iraqi Army's joint forces, said: "From yesterday to today there were more than 20 suicide cars that tried to target our vehicles during our advancement but they were taken care of by the heroes who man the Abram tanks and Hornets that target these suicide trucks. 

"We also have air support from the Iraqi air force. There haven't been any casualties but we managed to target and destroy them."

The attacks were a sign of the struggles that could face the troops when they reach Mosul.

Residents said sand berms and concrete walls were being used to close roads, while oil barrels and tyres were being put out to be set alight to obstruct visibility for US-led coalition aircraft. 

IS fighters have been using bicycles or motorbikes to get around the city to avoid being targeted from the air.

Those inside the city said they found out about the offensive from the radio and airstrikes had hit closer targets in recent days.

"The recent airstrikes are really shaking the ground and houses," one resident told Associated Press. 

"My wife prays and recites verses from the holy Quran when airstrikes start, while children cry. We are afraid that one of these airstrikes might hit us."