Boko Haram hostages were held in atrocious conditions in the group's Sambisa Forest stronghold, Nigeria's military said on Thursday after nearly 500 women and girls were released this week.

"Whoever they may be, the important thing is that Nigerians held captive under very severe and inhuman conditions have been freed by our gallant troops," defence spokesman Chris Olukolade said.

He told reporters in Abuja the army would "comprehensively" clean out the forest, adding: "There is great hope for the recovery of more hostages of the terrorists."