Amnesty International says Nigeria has reinstated a general implicated in mass murder, underlining the government's "monumental failure" to stamp out impunity for war crimes.

The London-based rights group called for an investigation last year of nine senior commanders, including Maj. Gen. Ahmadu Mohammed, for possible criminal responsibility for war crimes, including the deaths of more than 8,000 detainees since 2011. Mohammed was commanding officer when soldiers killed around 640 unarmed detainees after Boko Haram extremists attacked Giwa barracks in northeast Maiduguri city, according to Amnesty.

President Muhammadu Buhari in June promised to investigate the allegations and deal with all alleged abuses by the military, but nothing has been done.