KHAZER, Iraq (AP) — Islamic State militants have deployed suicide car bombs and fired mortar rounds to slow down the advance of Iraqi troops outside a key town near the militant-held city of Mosul, an Iraqi army officer said Wednesday.

The officer from the 9th Division told The Associated Press that his troops were now around 1 kilometer (half mile) away from Hamdaniyah, a historically Christian town also known as Bakhdida.

Since Tuesday, IS has sent 12 car bombs, all of which were blown up before reaching their targets, he said, adding that Iraqi troops suffered a small number of casualties from the mortar rounds. The officer, who spoke on condition of anonymity because he was not authorized to talk to reporters, did not provide specific figures.

Iraq launched a massive operation on Monday to retake Mosul, the country's second largest city and the extremist group's biggest urban bastion.

The operation is the largest launched by the Iraqi army since the 2003 U.S.-led invasion. Some 25,000 troops, including Sunni tribal fighters, Kurdish forces known as the peshmerga and state-sanctioned Shiite militias known as the Popular Mobilization Units are approaching the city from different directions.

The participation of the Shiite militias in the operation to retake the mainly Sunni Mosul has raised concerns that the campaign could inflame sectarian tensions. Rights groups have accused the Shiite militias of abuses in past campaigns against IS-held areas.

In a bid to alleviate those concerns, Shiite militia leaders on Tuesday announced that they will only focus on capturing the mostly Shiite town of Tal Afar to the west of Mosul, and not enter the city itself.

"The only troops who will enter Mosul are the army and police, not the Popular Mobilization Units or the peshmerga," said Hadi al-Amiri, the leader of the Badr Brigade, one of the largest Shiite militias.

"This has been agreed upon," he said at a press conference in the Shiite holy city of Najaf.

Amnesty International said in a report released Tuesday that Iraqi government and paramilitary forces detained, tortured or killed hundreds of Sunni Arab civilians fleeing IS-held areas during the operation to retake the Sunni city of Fallujah, west of Baghdad, earlier this year.

The Iraqi government has denied any systematic violations by security forces or the militias, and says individuals have been held accountable for occasional abuses.

Associated Press Writer Sinan Salaheddin in Baghdad contributed to this report.