The Islamic State group, also known as ISIS, has abducted almost 300 former members of the Iraqi Security Forces in the northern Iraqi city of Mosul, forcing 1,500 families to march from the village of Hammam al-Alil to Mosul, the United Nations said Tuesday.

Mosul has been under the control of the extremist group since 2014, and remains the last major city in the country to be held by ISIS. Last week, however, the Iraqi government forces, Kurdish peshmerga and Shiite militias, backed by U.S.-led airstrikes, made a strong push into the territory to regain control over Mosul after years of fighting.

“People forcibly moved or abducted, it appears, are either intended to be used as human shields or – depending on their perceived affiliations – killed,” said Ravina Shamdasani, spokeswoman for the U.N. High Commissioner for Human Rights, according to Reuters.

While almost 100 former officers were taken on Nov. 3 from Mawaly village, 12 miles west of Mosul, another 195 were abducted between Nov. 1 and Nov. 4 from different villages in Tal Afar district. The U.N. also cited reports that at least 30 sheikhs, or local leaders, were abducted in Sinjar district on Nov. 2 or Nov. 3.

“The fate of these civilians is unknown for the moment,” Shamdasani reportedly said at a briefing in Geneva.

“I cannot be exact about the number because we are still investigating,” said Col. Khalid Jaburi, a representative of the Iraqi council of ministers, according to the Guardian. Using the Arabic acronym for the group, he added: “Daesh also beheaded 100 former police officers and soldiers and dumped them in a mass grave.”

The mass grave was reportedly discovered after Iraqi troops called in a bulldozer after noticing a strong smell. Bones, decomposed bodies and scraps of clothing and plastics were uncovered on the site of the agricultural college where 50 police officers had also reportedly been killed last month.

Shamdasani, however, did not confirm these reports.

The military campaign, carried out by the Iraqi forces to rid the region of the ISIS, has entered its fourth week. With the forces being able to wrestle control of small areas in Mosul, ISIS has resorted to numerous beheadings and forced displacement of civilians to maintain its stronghold.