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Iraqi forces and pro-government militias have made gains into the Islamic State group stronghold of Tal Afar, as Prime Minister Haider al-Abadi announced the start of a campaign to take the town on Sunday.

Hashd al-Shaabi [Popular Mobilisation Forces] commanders said fighters had taken key hills surrounding the IS bastion, while others advanced into the town from the south.

"[We] advanced three kilometres from the southern direction toward Tal Afar and gained control on four hills that overlook the town from the same direction," a statement by the mostly Shia militia coalition published on Iraq News.

"These hills include Tal Zinbar, west of the town, which was freed by security troops along with the 26th Brigade," it continued.

The militia said Iraqi war planes destroyed armoured vehicles rigged with explosives as they attempted to drive at the pro-government forces.

Airstrikes also battered the IS frontlines in overnight air raids with "huge losses on the enemy", allowing federal police to seize more territory.

Around 2,000 IS militants are holed up in the city and with no escape they are expected to put up stiff resistance to the Iraqi forces' advance.

"Intelligence gathered shows clearly that the remaining fighters are mainly foreign and Arab nationals with their families and that means they will fight until the last breath," Colonel Kareem al-Lami told Reuters.

Prime Minister Haider al-Abadi announced the start of the campaign to take what has become IS' main stronghold in Iraq, following the fall of the country's second city Mosul.

There have been concerns about the involvement of Hashd al-Shaabi, who have been accused of mass killings and other human rights abuses during previous campaigns against IS held towns.

Despite the significant deployment of Hashd al-Shaabi fighters in the Tal Afar campaign, the US-led anti-IS coalition appear to back their involvement.

"Mosul was a decisive victory for the Iraqi Security Forces, but it did not mark the end of [IS] in Iraq, or its worldwide threat. The Iraqi Security Forces' operation to liberate Tal Afar is another important fight that must be won to ensure the country and its citizens are finally free of [IS]," said US Lt. Gen. Stephen Townsend, commander of the anti-IS coalition. 

"The [anti-IS] coalition is strong, and fully committed to supporting our Iraqi partners until [IS] is defeated and the Iraqi people are free." 

Townsend said the US-led coalition was providing Iraqi security forces with "equipment, training, intelligence, precision fires and combat advice" in the campaign.

The anti-IS coalition acknowledged the presence of the Hashd al-Shaabi in the campaign to retake Tal Afar.

"The [ISF] have proven themselves a capable, formidable, and increasingly professional force" the Operation Inherent Resolve statement read.

Tal Afar - a Shia-majority town before its 2014 capture by IS - has been a flashpoint for sectarian tensions since the 2003 US invasion.

Many of IS leadership are said to come from the town's Sunni population. The Hashd al-Shaabi - a coalition of mostly Iranian-backed Shia militias - have been accused of training children to fight at a military camp near Kirkuk.