The pro-government forces encountered trucks parked across roads with earthen embankments aimed at stopping them, as well as sniper fire from rooftops and mortar shelling.

Six weeks after routing the jihadists from Iraq’s second city Mosul, Iraqi forces launched an assault Sunday on Tal Afar, where an estimated 1,000 jihadists are holed up.

They first retook three districts of the city on Tuesday, but as with the gruelling nine-month campaign to recapture Mosul, their convoys face an onslaught of suicide and car bomb attacks.

On Wednesday, they “entered the neighbourhood of Al-Kifah North and headed towards the centre of the city,” said Ahmed al-Assadi, spokesman for the Hashed al-Shaabi battling IS alongside the army and police.

“All the lines of IS defence outside the city have been broken and the troops are advancing from all directions towards the inner quarters of the city,” he added.

The Hashed also announced the capture of the districts of Al-Tanak and Al-Sinaai in eastern Tal Afar.

As government forces advanced, troops said they discovered a network of underground tunnels used by the jihadists to launch attacks behind lines of already conquered territory, or to escape.

In a bid to counter these surprise attacks, the Iraqis dropped leaflets overnight calling on civilians to help by marking houses where the jihadists are located.

The International Organisation for Migration said “thousands of civilians” had fled Tal Afar since the offensive began.

But around 30,000 civilians are trapped in the fighting, according to the United Nations.

Caught between the two sides, those still inside the city have been pounded by Iraqi and US-led coalition aircraft for weeks, as well as intense artillery fire since Sunday.

The UN refugee agency (UNHCR) fears they could be “used as human shields” and that “attempts to flee could result in executions and shootings,” said the spokesman for UN Secretary General Antonio Guterres. The United Nations and aid agencies are working to establish shelters for the displaced.

Those who flee through desert areas face temperatures of up to 43 degrees Celsius (109 Fahrenheit), sometimes for periods of more than 10 hours, putting them at risk of dehydration, said Viren Falcao of the Danish Refugee Council.

Tal Afar was once a key supply hub for IS between Mosul, which lies around 70 kilometres (45 miles) to the east, and the Syrian border.

The Iraqi forces massed around Tal Afar on Tuesday before the jihadists responded with artillery fire.

Army, police and of the Hashed al-Shaabi paramilitary coalition later took “full control” of the Al-Kifah, Al-Nur and Al-Askari districts, the Hashed said

The Iraqi forces had encircled the city despite what Hashed spokesman Assadi called “intense” fighting. He said the battle for the city would probably last weeks, in contrast to the months-long battle for Mosul.

After meeting Iraqi Prime Minister Haidar al-Abadi in Baghdad on Tuesday, US Defence Secretary Jim Mattis said the jihadists were “on the run”.

“Cities have been liberated, people freed from ISIS, from Daesh,” Mattis said, using alternative names for IS.

The jihadists had not been able “to stand up to our team in combat, and they have not retaken one inch of ground that they lost,” he said.

IS jihadists in June 2014 overran Tal Afar, a Shiite enclave in the predominantly Sunni province of Nineveh.

At the time, its population of around 200,000 was overwhelmingly Turkmen, one of Iraq’s largest ethnic minorities.

Tal Afar’s Shiites were directly targeted by IS, while some members of its Sunni minority joined the jihadists and went on to form a contingent with a particularly brutal reputation.