BAGHDAD (Reuters) - Iraq is readying a special force tasked with storming Tal Afar, a bastion of Islamic State west of Mosul, in a way that would avoid revenge killings against the town’s Sunni population.

Shi'ite fighters fire a rocket towards Islamic State militants during a battle with Islamic State militants at the airport of Tal Afar west of Mosul, Iraq November 18, 2016. REUTERS/Thaier Al-Sudani

Capturing or isolating Tal Afar from Mosul is a key part of the U.S-backed offensive to take back Islamic State’s last major city stronghold in Iraq, by allowing the ground forces taking part in the campaign to complete Mosul’s encirclement.

The army and police forces planning to storm Tal Afar will include Sunni and Shi’ite Turkmens, reflecting the main mix of the town’s population, said Hisham al-Hashimi, a Baghdad-based analyst who advises the government on Islamic State affairs.

Tal Afar’s Shi’ite community fled after Islamic State, a hardline Sunni group, swept through the region two years ago, declaring from Mosul a “caliphate” that also spanned parts of Syria.

The offensive to take Mosul started on Oct. 17 with air and ground support from a U.S.-led coalition.

The attack on Tal Afar has been so far executed by Popular Mobilisation, a coalition made up mainly of Iranian-trained Shi’ite paramilitary groups whose involvement in the Mosul offensive sparked fears of sectarian killings.

The 3,500-strong force being set up to storm Tal Afar will operate from an air base just south of the city, said Hashimi. Popular Mobilisation, or Hashid Shaabi, will stay outside the town, enforcing a blockade, he said.

The Iraqi government has been keen on allying fears of sectarian killings in Tal Afar after Turkey threatened to intervene, citing its historic ties to Iraq’s ethnic Turkmens.

FLEEING CIVILIANS

Thousands of civilians fled Tal Afar this week as Popular Mobilisation closed in.

Several dozen families from the town also arrived in Mosul, said a city resident, asking not to be identified as Islamic State is punishing by death those caught communicating with the outside world.

Mosul is already ringed to the north, south and east by Iraqi government forces and Kurdish peshmerga forces. Iraq’s U.S.-trained Counter Terrorism Service breached Islamic State defenses in east Mosul at the end of October.

Islamic State fighters in Mosul are dug in among more than a million civilians as a tactic to hamper air strikes. They are resisting the advancing troops with suicide car bombs and sniper and mortar fire.

“There are lots of civilians and refugees, we cannot use our tanks,” said a Counter Terrorism officer, Fadhil Barwari. “We take one neighborhood and people are leaving, we can’t work out who is who,” he added. “They (Islamic State) are firing rockets and they are hitting civilians.”

Islamic State has also stepped up a bombing campaign in government-held areas in a attempt to relieve pressure on Mosul.

On Thursday, it claimed a massive truck bomb near Hilla, hundreds of kilometers south of the front lines, killing about 100 people.

Iranian President Hassan Rouhani on Friday called on the Iraqi government to keep up the campaign against the “terrorists whose hands are drenched in blood”.

The Iraqi military estimates there are 5,000 to 6,000 insurgents in Mosul facing a 100,000-strong coalition force.

Islamic State leader Abu Bakr al-Baghdadi, believed to be somewhere near the Syrian border, has told his fighters there can be no retreat.