Islamic State militants have taken 550 families from villages around Mosul and are holding them close to Islamic State group locations in the Iraqi city, probably as human shields, the U.N. human rights office said on Friday.

"We are gravely worried by reports that ISIL is using civilians in and around Mosul as human shields as the Iraqi forces advance, keeping civilians close to their offices or places where fighters are located, which may result in civilian casualties," U.N. High Commissioner for Human Rights Zeid Ra'ad al Hussein said in a statement.

"There is a grave danger that ISIL fighters will not only use such vulnerable people as human shields but may opt to kill them rather than see them liberated."