Iraqi forensic experts are investigating a mass grave that was discovered by troops advancing towards the Islamic State-held city of Mosul.

The Iraqi military has said the grave, in the grounds of an agricultural college in the town of Hamam al-Alil, contains about 100 decapitated bodies. Most have been reduced to skeletons, so it is unclear who the victims were.

IS militants have carried out many mass killings and have been accused of fresh atrocities in the area in recent weeks. They are being forced to retreat to Mosul, the last major IS urban stronghold in Iraq, in an offensive by pro-government forces that began on 17 October.

Iraqi troops found the mass grave after noticing a strong smell while advancing into the town, about 30km (19 miles) south of Mosul, on Monday.

It was not immediately known if the victims were security forces personnel or civilians. But video footage from the Associated Press showed a soldier holding up a child’s stuffed animal found at the grave.

A member of the Nineveh provincial council, Abdul Rahman al-Waggaa, said IS militants had used the agricultural college as “a killing field” for hundreds of people in the days before troops, police and militiamen retook the town. “They would torture them inside and then take them out of the neighbourhood and either shoot them or slit their throats,” he told Reuters news agency.

A former English teacher from Hamam al-Alil, Riyad Ahmed, meanwhile said he had seen IS militants dragging civilians to a makeshift jail near his home and then sending them in the middle of the night to be killed.

In 2014, the jihadists carried out mass killings of security forces personnel and members of ethnic and religious minorities as they swept across northern Iraq after taking control of Mosul, boasting about them in photos and videos circulated online.

Since the operation to recapture Mosul was launched three weeks ago, the UN has received reports that militants have carried out fresh atrocities. On Tuesday, the Office of the UN High Commissioner for Human Rights said it was investigating whether the grave at Hamam al-Alil was connected to the alleged killing of 50 former police officers at the same agricultural college last month.

Spokeswoman Ravina Shamdasani added that IS militants had also reportedly abducted 295 former Iraqi Security Forces personnel areas west of Mosul in the past week, and forcibly transferred 1,500 families from Hamam al-Alil to Mosul airport with the apparent intention to use them as human shields.

Also on Tuesday, Kurdish Peshmerga fighters reportedly took complete control of the town of Bashiqa, about 12km north-east of Mosul, a day after launching an assault to retake it. Jabar Yawar, secretary-general of the Kurdistan Regional Government’s ministry of Peshmerga affairs, told AFP news agency that forces were sweeping the town for militants hiding inside buildings and tunnels. At least 13 were killed while trying to escape on Tuesday, he said.

Meanwhile, Iraqi special forces and army units continued to clear eastern districts of Mosul, a week after they entering the city for the first time in two years.