QAYYARAH (Iraq) • Iraqi paramilitary forces battled the Islamic State group south-west of Mosul yesterday, the second day of an operation to cut supply lines of extremists between the city and neighbouring Syria.

Tens of thousands of Iraqi troops and Kurdish peshmerga fighters have been advancing on Mosul from the north, east and south after the launch on Oct 17 of a vast offensive to retake the last stronghold of the Islamic State in Iraq and Syria (ISIS) in the country.

After standing largely on the sidelines in the first days of the assault, forces from the Hashed al-Shaabi - a paramilitary umbrella organisation dominated by Iran-backed Shi'ite militias - began a push on Saturday towards the west of Mosul.

The involvement of the Shi'ites could whip up sectarian and regional tensions in an already complex battle.

The ultimate aim of the push is the recapture of Tal Afar, a town west of the city, and the cutting of ISIS supply lines between Mosul and Syria, but the Hashed still have significant ground to cover.

The United Nations says there have been credible reports of ISIS carrying out mass executions in the city and seizing tens of thousands of people for use as human shields.

In a series of statements during the weekend, the Hashed media office said it had retaken two villages, cleared another area and entered several more. Al-Imraini, one of the two villages the Hashed said it recaptured, is 45km from Tal Afar, according to the media office.

The drive towards Tal Afar could bring the fighting perilously close to the ancient city of Hatra, a Unesco world heritage site, and the ruins of Nimrud - two archeological sites that have previously been vandalised by ISIS.

The involvement of Shi'ite militias in the Mosul operation has been a source of contention, though the Hashed's top commanders insist they do not plan to enter the largely Sunni city.

Iraqi Kurds and Sunni Arab politicians have opposed their involvement, as has Turkey, which has a military presence east of Mosul despite repeated demands by Baghdad for the forces to be withdrawn.

Relations between the Hashed and the United States-led coalition fighting ISIS are also tense, but the paramilitaries enjoy widespread support among members of Iraq's Shi'ite majority.

The US-led coalition - which has been assisting federal forces and Kurdish peshmerga with air strikes, training and advisers for two years - said on Friday that Iraqi forces were observing a pause in the two-week-old offensive.

In Bartalla, a Christian town just east of Mosul, army and counter-terrorism forces were consolidating their positions, unloading cases of weapons from trucks and organising their ammunition stocks.

AGENCE FRANCE-PRESSE