Amnesty International is calling for an investigation after officials said Nigeria's military had secretly buried more than 300 Shia Muslims in a mass grave.
Officials in Kaduna state told an inquiry that soldiers took bodies from a morgue to a bush site following a crackdown on a pro-Iranian sect.
The raid in December followed accusations the sect had tried to assassinate the army chief.
The sect denies the allegation.
In December, troops besieged the home of the leader of the Islamic Movement of Nigeria (IMN) Sheikh Ibraheem Zakzaky in the northern city of Zaria.
The IMN alleges that the military killed hundreds of its members and destroyed a religious shrine and the sheikh's home during the raid.
Protests broke out in five cities across Nigeria, while Iran's President Hassan Rouhani phoned his Nigerian counterpart, Muhammadu Buhari, about the crackdown.
An inquiry heard on Monday that the Kaduna State High Court gave permission to bury the bodies.
Officials said 347 bodies were moved.
The rights group Amnesty wants to protect the burial site and is campaigning for a full forensic investigation following what it called "horrific revelations of the slaughter and secret burial".
Shias in Nigeria
- Shias are minority in Nigeria but their numbers are increasing
- The IMN, formed in the 1980s, is the main Shia group led by Sheikh Ibraheem Zakzaky
- They operate their own schools and hospitals in some northern states
- They have a history of clashes with the security forces
- The IMN is backed by Shia-dominated Iran and its members often go there to study
- Sunni jihadist group Boko Haram condemns Shias as heretics who should be killed