Amnesty International is calling for an investigation of military killings of Nigeria's minority Shiite Muslims following revelations of hundreds buried in a mass grave.

Kaduna state government secretary Balarabe Lawal told a commission of inquiry this week that dozens of soldiers and state officials transported 347 bodies from a mortuary and an army base to a remote site where they were buried after the Dec. 12-14 military raids on Shiite compounds in northern Zaria town.

The Shiites say those killed number closer to 1,000.

The military said it ordered the raids after Shiites allegedly attempted to assassinate Nigeria's army chief.

London-based Amnesty said those responsible for the "atrocity" must face justice. It said scores of Shiites remain in unlawful detention including leader Ibraheem Zakzaky.