Iraqi paramilitary forces said they had captured several villages southwest of Mosul from the Islamic State group on Sunday, the second day of an operation to cut the jihadists' supply lines.

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 October 17 of a vast offensive to retake IS's last stronghold 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 Shiite 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 severing of jihadist supply lines between Mosul and Syria.

In a series of statements on Sunday, the Hashed's media office announced it had retaken at least four villages southwest of Mosul.

Al-Imraini, one of the recaptured villages, is 45 kilometres (27 miles) 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 IS.

The involvement of Shiite 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.

- Concerns over militias -

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as has Turkey which has a military presence east of Mosul despite repeated demands by Baghdad for the forces to be withdrawn.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 IS are also tense, but the paramilitaries enjoy widespread support among members of Iraq's Shiite majority.

The Hashed has been a key force in Iraq's campaign to retake areas seized by IS in mid-2014, when the jihadists 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 IS, including summary killings, kidnappings and destruction of property.

Tal Afar was a Shiite-majority town of mostly ethnic Turkmens before the Sunni extremists of IS overran it in 2014, and its recapture is a main goal of Shiite 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.

- Over 17,600 displaced -

The US-led coalition -- which has been assisting federal forces and Kurdish peshmerga with air strikes, training and advisers for two years -- said 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 Organization 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 IS carrying out mass executions in the city and seizing tens of thousands of people for use as human shields.

IS'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 jihadists are "effectively using tens of thousands of women, men and children as human shields," he said.

The UN cited reports indicating IS 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.