The UN statement late on Monday condemned the killing as "particularly abhorrent", without providing further details. The six were captured on October 28 during an attack on the town of al-Fuqaha, AP reported.
The town is south of the coastal city of Sirte, a former Daesh-held bastion.
The attack underscored the volatility of Libya's central region, where Daesh and other armed groups, including some from neighboring Chad, operate as highway robbers or attack patrols of the self-styled Libyan National Army.
Libya plunged into chaos after the 2011 uprising and is now governed by rival administrations based in country's east and west.