The U.N.'s human rights chief says his office has received reports that Boko Haram fighters fleeing advancing military forces murdered women and girls they had taken as "wives," along with other captives.

Zeid Raad al-Hussein told a special session of the U.N. Human Rights Council in Geneva Wednesday that the recent recapture of parts of northeastern Nigeria "has brought to light gruesome scenes of mass graves and further evident signs of slaughter by Boko Haram."

He didn't give further details of the reported killings of fighters' "so-called 'wives' — in fact, women and girls held in slavery."

Zeid said there also are "persistent and credible reports" of serious rights violations by Nigerian and other security forces responding to Boko Haram. He called for "thorough and fully transparent investigations" by authorities.