Citing credible reports received by her office, Ravina Shamdasani, spokeswoman for the UN refugee agency, said 295 people had been kidnapped in Tal Afar, a drive of about 77 kilometers (47 miles) west of Mosul.
"The fate of all 295 civilians is unknown," she said.
Shamdasani said the citizens had been targeted because they were former Iraqi security forces personnel, now living under ISIS as noncombatants.
Eighteen of the town's 30 sheiks have been killed, according to a report received by the United Nations.
Shia militia have been advancing on Tal Afar
, a largely ethnic Turkmen town, to cut off an avenue of retreat to Syria for ISIS militants who may be trying to flee Mosul.
Mass grave found
The reports of the abductions and killings in Tal Afar come in the wake of the discovery of a mass grave outside Hammam al-Alil
, a town south of Mosul recaptured Monday from ISIS.
Iraqi forces found the grave, containing the remains of about 100 beheaded civilians, at the School of Agriculture on the outskirts of Hammam al-Alil, Iraq's Joint Military Command said.
"Gangs of ISIS militants continue to commit crimes against our people," the statement said.
Beheadings and mass graves have become a hallmark of the Sunni terror group.
Iraq's federal police forces and army troops attacked Hammam al-Alil
-- about 30 kilometers (18.6 miles) south of Mosul and the last significant settlement on the Tigris River before reaching that city -- on Saturday.
A day earlier, the UN refugee agency said it had received reports that ISIS was instructing residents of Mosul to hand over boys 9 and older
, apparently to fight on behalf of the terror group.
Tens of thousands displaced
Since October 17, an Iraqi-led coalition has been waging an offensive to retake Mosul, ISIS' last major stronghold in Iraq.
Mosul, Iraq's second-most populous city with 1.2 million residents, has been under the terror group's control for two years.
Aid agencies have warned the offensive could trigger a refugee crisis; so far, 34,860 people have been displaced, according to the International Organization for Migration.
Coalition forces have made swift progress on the approach to Mosul but are encountering fierce resistance on entering the city.
The estimated 3,000 to 5,000 ISIS fighters in the city have had two years to prepare for the Iraqi forces' arrival. Forces entering Mosul are expecting extensive networks of tunnels and booby-trapped buildings.
Peshmerga move on Bashiqa
Kurdish Peshmerga forces, which are key in the fight against ISIS, have been working to drive the remaining terror elements from Bashiqa, a few miles northeast of Mosul, after a three-pronged assault on the town Monday.
A senior Peshmerga general, along with two soldiers, were killed Tuesday in an ISIS ambush in the town, according to Brig. Gen. Behram Doski, Peshmerga commander in Bashiqa.
Peshmerga forces were searching house to house for hidden militants when nine ISIS fighters ambushed them, Doski said. The ISIS militants were later killed.
Peshmerga Maj. Gen. Ahmed Koei told CNN he believed 50 ISIS fighters had been killed in the battle for Bashiqa, with some still remaining behind in houses while scores of others are believed to have fled.