The militants say Henry Okah's health has been failing
Nigerian militant leader Henry Okah has accepted a government amnesty offer, his lawyer says - possibly a step towards peace in the oil-rich Delta.
President Umaru Yar'Adua was "elated" at the news, his spokesman said on the sidelines of the G-8 summit in Italy.
Mr Okah is facing treason and gun-running charges since his arrest in Angola in 2007. His release has been a key demand of his Mend militant group.
Attacks in the Niger Delta have severely cut Nigeria's oil output.
In recent days, the Movement for the Emancipation of the Niger Delta (Mend) says it has blown up several oil pipelines and has captured six foreign crew from onboard an oil tanker.
Senior Mend official "General" Boyloaf confirmed to the BBC that, if Mr Okah was set free, the organisation would lay down its arms.
But he said the release of Mr Okah was just one issue - and there were other issues to be settled in the region.
Another spokesman, Jomo Gbomo, sent an e-mail to news organisations supporting Mr Okah's decision, saying his health was failing.
But he said the group does not believe the amnesty offer is aimed at "freedom fighters", as it does not allow for negotiation.
However, the statement does welcome the appointment of Timi Alaibe as the special adviser to the president on Niger Delta affairs, saying he is a man the militants "respect and can work with".
Presidential spokesman Olusegun Adeniyi said the attorney general was working with Mr Okah's lawyer on the details of his release.
The government recently offered an amnesty to members of any militant group which laid down its weapons.
The Mend rebels have been fighting for an increased share of Nigeria's vast oil wealth in the Niger Delta region, but the government has in the past dismissed them as criminals.
The militants say they are fighting for the rights of local people in the delta.
The government's amnesty offer is part of an effort to end years of rebel attacks on the Nigerian oil industry. Oil output has suffered because of such attacks.
The government has said militants who give up their weapons by October will benefit from a rehabilitation programme, including educational and training opportunities.
Correspondents say that even if Mr Okah is released, it is not clear whether there will be peace in the region.
They say that because the unrest in the Niger Delta is not a straightforward political struggle, there are doubts whether the amnesty alone will be enough to halt opportunistic attacks, oil theft and kidnapping.