Boko Haram extremists have returned most of the 110 girls abducted from their Nigeria boarding school a month ago with an ominous warning, witnesses said.

Nigeria's government has acknowledged that 110 girls are still missing nearly a week after Boko Haram militants attacked a town.

Some 21 of the Chibok schoolgirls kidnapped by Boko Haram Islamic extremists more than two years ago have been freed in negotiations, officials said.