(CNN) -- Nearly 1,000 people, mostly members of the Islamist extremist group Boko Haram, have died this year while in Nigerian custody, Amnesty International said in a report.
Nigerian officials strongly denied the accusations in the report, which was released Tuesday.
According to Amnesty, the human rights group has received credible information about deplorable conditions in detention centers, where prisoners are said to die daily.
The report focuses on three detention centers where suspected militants are sent.
Overcrowding results in suffocation and starvation, Amnesty said. Others have died after suffering beatings or being shot and left to bleed to death, according to former detainees interviewed by the group.
Nigerian officials dismissed the report as a fabrication.
"There is no way our government officials will take the lives of others," Interior Minister Abba Moro told CNN.
The facilities in question are run by Nigeria's Joint Task Force.
"These claims are false," said Eli Lazarus, spokesman for the task force. "We will conduct a thorough investigation if necessary and release a statement."
Amnesty called on the government to conduct an investigation "as a matter of urgency."
The two facilities were a majority of the deaths are said to have happened are the Giwa military barracks in Borno state and two centers known as Sector Alpha and Presidential Lodge in Yobe state.
Boko Haram has waged an insurgency in the nation for years. The militant group has attacked various targets in the nation since 2009, murdering and kidnapping Westerners, and bombing schools and churches.