Around 100 beheaded bodies were found in a mass grave in a school near Mosul, the Iraqi military said on Monday, as forces continue to advance south of ISIS' last remaining stronghold in the country.

According to CNN, the grave was found in the School of Agriculture, a college on the outskirts of the town of Hammam al-Alil which was liberated from ISIS yesterday.

A statement from the Iraqi Joint Command condemned the killings, saying: "Gangs of ISIS militants continue to commit crimes against our people".

Colonel Abdel Rahman Khazali, of the Iraqi federal police, said an investigation had begun into the identification of the bodies.

A large-scale offensive to take back Mosul, which was overrun by ISIS militants more than two years ago, began on October 17.

Around 34,000 people have been displaced by the offensive in the weeks since and hundreds of thousands more are expected to flee the violence.

The UN Refugee Agency warned on November 4 that sources suggested boys as young as nine were being taken by ISIS to fight alongside jihadis as the push intensified.

ISIS is known to have committed massacres, and has documented them for propaganda use.

When militants took over the Sinjar region in August 2014, up to 5,000 Yazidi men died in a series of mass killings and 30 mass graves have since been discovered in liberated areas to the north of Sinjar Mountain.

The systematic persecution of religious groups including Yazidis, Christians and Shia Muslims by ISIS has been labelled a genocide by the US administration, European Parliament, UK Parliament and Council of Europe.