QAYYARAH, IRAQ (AFP) - Iraqi paramilitary forces battled the Islamic State group (ISIS) south-west of Mosul on Sunday (Oct 30), the second day of an operation to cut the militants' supply lines 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 a vast offensive to retake ISIS's last stronghold in the country was launched on Oct 17.

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 ultimate aim 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 has significant ground to cover.

In a series of statements on Sunday, the Hashed's 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 toward 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as has Turkey which has a military presence east of Mosul despite repeated demands by Baghdad for the forces to be withdrawn.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 US-led coalition fighting ISIS are also tense, but the paramilitaries enjoy widespread support among members of Iraq's Shi'ite majority.

The Hashed has been a key force in Iraq's campaign to retake areas seized by ISIS in mid-2014, when the extremists took control of large parts of Syria and Iraq and declared a cross-border "caliphate".

But the paramilitaries have been repeatedly accused of human rights violations during the course of the war against ISIS, including summary killings, kidnappings and destruction of property.

Tal Afar was a Shi'ite-majority town of mostly ethnic Turkmens before the Sunni extremists of ISIS overran it in 2014, and its recapture is a main goal of Shi'ite militia forces.

The Sunday fighting came a day after Iraq announced the recapture of Al-Shura, an area south of Mosul with a long history as a militant bastion that has been the target of fighting for more than a week.

Iraq's Joint Operations Command announced "the complete liberation of Al-Shura", saying that security forces advancing from four different sides had linked up in the area, which is north of Qayyarah base, the main hub for the southern front.

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.

More than 17,600 people have fled their homes toward government-held areas since the Mosul operation began, the International Organisation for Migration said on Sunday.

Numbers are expected to soar as Iraqi forces close in on the city, which is home to more than a million people.

The UN 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.

ISIS's "depraved, cowardly strategy is to attempt to use the presence of civilians to render certain points, areas or military forces immune from military operations", UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein said in a statement.

The extremists are "effectively using tens of thousands of women, men and children as human shields", he said.

The UN cited reports indicating ISIS has forcibly taken civilians into Mosul, killing those who resist or who were previously members of Iraqi security forces.

It said more than 250 people were executed in just two days earlier this week.