Amnesty International says it has "credible reports" that Nigerian authorities hanged four convicted criminals in its first known executions since 2006.

The London-based human rights organization says a fifth man whose death warrant has been signed by the governor of Edo state in southern Nigeria is in at "imminent risk of execution."

Amnesty's Deputy Africa director Lucy Freeman said such executions would mark "a truly dark day for human rights" in Africa's most populous nation.

Amnesty said more than 1,000 people are reportedly on death row in the country.

It was not immediately possible to verify the report that the four were hanged Monday at Benin City Prison or to get government comment on whether a moratorium has ended.