Nigeria on Wednesday said it would open a probe into claims of state-sponsored killings dating back to the era of military rule, raising hopes that perpetrators will finally be brought to book.

The National Human Rights Commission (NHRC) said a public inquiry would be held into complaints it had received "bordering on allegations of state-sponsored killings or assassinations".

A newspaper advertisement to announce the probe said it would cover the period from November 1995, when Nigeria was under military rule, to the present day civilian administration.

The formal announcement comes after the military ruler turned civilian head of state Olusegun Obasanjo accused President Goodluck Jonathan of training a private army to carry out political "hits".

Jonathan denied the charge, which was made last December in an 18-page open letter criticising his administration, but ordered an investigation.

Claims will have to be submitted for consideration within three weeks and hearings would be held on a date to be fixed, the NHRC said.

The organisation's chairman, Chidi Odinkalu, said the inquiry was not a witch-hunt but was designed to deliver justice for those affected by alleged abuses.

"Let's get the issue straight. The investigation is not about individuals. This is a story of victims that don't have remedies," he told AFP.

"It's about institutions that may have committed rights abuses."

Amnesty International praised the move as a "good initiative" while Human Rights Watch said it was "an important step in ensuring accountability for the many unresolved killings in Nigeria since 1995".

Both groups said they had documented a number of cases of extra-judicial and politically motivated killings over the years, particularly around the time of elections.

"We hope that the National Human Rights Commission will be given unhindered support to carry out its investigation adequately and independently," said Amnesty's West Africa researcher Makmid Kamara.

"We trust that the people who are in charge will be given the scope to execute their functions," he added in a phone interview from London.

But both Amnesty and HRW said they hoped that any eventual recommendations would lead to concrete action, unlike previous, similar investigations that have led nowhere.

Under Obasanjo, who became civilian president in May 1999, a panel was set up to investigate cases of rights abuses and violations, including politically motivated murders under previous regimes.

But the inquiry's report was never made public.

In December 2001, Obasanjo's justice minister Bola Ige was killed at his home in Ibadan, southwest Nigeria, while a key opposition figure, Harry Marshall, was murdered in his home in Abuja.

Under the military dictator Sani Abacha, the wife of Moshood Abiola, the presumed winner of the 1993 presidential election annulled by the military, was shot dead in Lagos.

Several pro-democracy activists and opponents of the Abacha regime were also killed.

Barely a month after Abacha's death in June 1998, Abiola died unexpectedly in custody during negotiations for his release.

Nigeria, Africa's most populous country of 170 million people, goes to the polls to elect a new president and parliament on February 14 next year.

Tensions are already rising in the run up to the crucial vote in which Jonathan is expected to seek re-election.

HRW said it hoped the findings of the NHRC inquiry "will inform concrete action that will send the right signals particularly on the imperatives of violence-free elections".

Kamara added: "We hope it will send a message to politicians and political parties that there is no place for extra-judicial killing of... opponents in the democratic dispensation of the country."