A Nigerian state official has confirmed that at least seven people have been executed in recent years, despite denials by the national government.
Nigeria retains the death penalty as a "deterrent"
Amnesty International said the executions had taken place in secret after convictions in Kano State.
Last month a Nigerian representative at the United Nations said: "Nigeria has not carried out any capital punishment in recent years."
A Kano official told the BBC that armed robbers could face the death penalty.
Kano governor's spokesman Sule Yau Sule told the BBC's Focus on Africa programme that the cases had been taken all the way to Nigeria's Supreme Court under the previous government.
He also said that Kano State would be willing to carry out further executions "as long as due process" had been followed.
According to the UN's official record of the committee debate on a death penalty moratorium on 15 November, the representative of Nigeria said the death penalty was still on his country's statute books for national security reasons and as a deterrent against serious crimes.
The Nigerian representative, whose name was not given, said "punishment only comes after exhaustive legal and judicial processes, including recourse to the Supreme Court of the land."
The moratorium proposal will be put to a full vote in the 192-member UN general assembly on 18 December and Nigeria has already indicated it will vote against.
In a statement sent by Amnesty to media organisations, Africa programme director Erwin van der Borght said: "The Nigerian government has been misleading the world".
He called on Nigeria to "come clean on their death penalty record, establish an immediate moratorium on all executions in the country and fully investigate."
Amnesty said the seven men had all been convicted in Kano State, before being transferred elsewhere in the country for their executions:
- Kenneth Ekhone and Auwalu Musa, executed at Kaduna Central Prison, May 2006.
- Salisu Babuga, executed at Jos prison, June 2006
- Reported hangings of four men in Enugu and one in Port Harcourt
In May this year a Nigerian presidential commission report called for "an official moratorium on executions until the Nigerian criminal justice system can ensure fundamental fairness and due process in capital cases."
The report also highlighted that inmates on death row are "almost exclusively poor and without legal representation."