Nigeria executes prisoners for first time since 2006
Four prisoners have been hanged in southern Nigeria, officials say, in what are believed to be the first executions for seven years.
The justice commissioner for the state of Edo, Henry Idahagbon, told reports that the prisoners had been convicted of armed robbery or murder.
Amnesty International said the executions represented "a truly dark day for human rights" in Nigeria.
More than 1,000 prisoners in Nigeria are believed to be on death row.
Local media say President Goodluck Jonathan recently asked state governors to sign death warrants in an effort to reduce crime.
Mr Idahagbon said the four men were hanged at Benin City Prison after a court had ordered their executions on Monday afternoon.
He said all their appeals had been exhausted and their death warrants had been signed - two by Edo Governor Adams Oshiomhole, and the others by governors of other states.
The executions were carried out by the prison service, a federal agency, and not the Edo state government.
There were conflicting reports about the fate of a fifth prisoner.
Chino Obiagwu of the Legal Defence and Assistance Project (LEDAP) told the Associated Press the man had been killed.
The execution was initially delayed by technical problems with the gallows, but he was informed later by email that it had gone ahead, Mr Obiagwu said.
But Mr Idahagbon said the man had not been executed because the prisoner's sentence had called for him to be killed by firing squad, which the prison authorities were not prepared to do.
Amnesty International said it had received credible reports of four executions in Benin City.
"If confirmed, these executions mark a sudden, brutal return to the use of the death penalty in Nigeria, a truly dark day for human rights in the country," its deputy director for Africa, Lucy Freeman, said in a statement.
She called on the Nigerian authorities to "return to the moratorium on executions".
Although Monday's executions would be the first known to have taken place since 2006, Nigerian security forces have been accused repeatedly of carrying out extra-judicial killings in the intervening years.