Three convicted criminals on death row for about two decades have been secretly executed in the first hangings in Nigeria since 2013, human rights lawyers and a fellow inmate said Thursday.

The executions breach a seven-year moratorium on the death penalty in this West African country. The hangings went ahead despite outstanding appeals, making them "unlawful killings," Chino Obiagwu of the Legal Defense and Assistance Project told The Associated Press.

Debate about the death penalty has revived recently in Nigeria, with some calling for people convicted of gross corruption to get a death sentence.

Obiagwu said the three inmates were hanged in Benin City Prison in southern Edo state on Dec. 23. All had been convicted of armed robbery by military tribunals in the 1990s, when Nigeria was under military dictatorship, security forces and prison officials regularly tortured confessions from suspects and there was no right of appeal. Armed robbery carried a mandatory death sentence.

Nigeria has more than 1,000 prisoners on death row.

Human rights lawyers were alerted when Nigeria's sole remaining executioner was summoned from northern Sokoto city to Benin, Obiagwu said. Contacts in the prison told them the executioner was oiling the gallows, and two of the three men were transferred from northern prisons.

Obiagwu said his organization wrote to Edo state Gov. Godwin Obaseki on Dec. 21 warning the men had outstanding appeals.

Officials in Edo did not immediately respond to requests for comment.

Edo state executed four convicted criminals in 2013, breaching the country's 2009 moratorium, until a public outcry stopped more hangings.

An inmate on death row told the AP that the men in the latest executions had been boasting to other prisoners that they believed they were about to be freed after lengthy incarcerations - Ogbomoro Omoregie for 25 years, Apostle Igene for 21 years and Mark Omosowhota for 19 years.

But then the inmate saw them being taken from their cells on Dec. 22. The inmate spoke on condition of anonymity because he is not supposed to have contact with the outside world.

The executions did not take place until dawn on Dec. 23 because there was no electricity to power the gallows, Obiagwu said. He said he waited to publish the news until he had confirmed the deaths with a medical team that removed the bodies.

The inmate said tensions were running high in the prison, with people scared there will be more executions. He said more than 50 people are on death row there.

AP