The abductions took place last week as Iraqi government forces, Kurdish peshmerga and Shi'ite militias backed by US-led air strikes pushed an offensive to recapture Mosul from Islamic State.
"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 UN High Commissioner for Human Rights.
About 100 of the former ISF officers were taken at around midnight on Nov. 3 from Mawaly village, which is about 20 km (12 miles) west of Mosul.
A further 195 were abducted between Nov. 1 and Nov. 4 from villages in Tal Afar district.
The abducted families were being taken from their town to Mosul airport, Shamdasani said.
"The fate of these civilians is unknown for the moment," she told a UN briefing in Geneva.
The United Nations also had information that at least 30 sheikhs were abducted in Sinjar district on Nov. 2 or Nov. 3 and taken to an unknown location.
It was trying to verify a report that 18 of them had been killed on Nov. 4 in Tal Afar district, Shamdasani said.
The operation against Islamic State's Iraqi stronghold has entered its fourth week and Iraqi forces have so far gained just a small foothold in Mosul.
The UN human rights office has sources on the ground but the information they are able to provide is "patchy", Shamdasani said.
She could not confirm media reports of a mass grave being found but said it happened to be in the same agricultural college in Hammam al Alil where the UN reported the execution of 50 police officers last month.
Related reading
Syrian militias make progress in IS-Raqqa
