Four prisoners have been hanged in Nigeria, an official said on Tuesday, in the first known executions since 2006 which Amnesty International described as "a truly dark day for human rights in the country."

Nigeria has faced international criticism over its death penalty and an estimated 1,000 people are on death row.

The four prisoners were convicted of either armed robbery or murder and were hanged in southern Edo state on Monday, the state's justice minister told Reuters by phone.

"They have been on death row for a long time and they were executed yesterday," Henry Idahagbon said. He did not say why authorities had decided to use the death penalty after a gap of seven years.

"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," said Lucy Freeman, Amnesty International's Deputy Director for Africa.

Idahagbon said two of the death warrants had been signed by Edo State Governor Adams Oshiomhole, and the other two by the previous governor.

"If the international community deems it wrong they should approach the national assembly for review of the law," he said.