A prominent Niger Delta militant leader, Henry Okah, has been extradited from Angola to Nigeria, according to officials in Angola.
Nigerian militants have been responsible for kidnappings
A founder of the Movement for the Emancipation of the Niger Delta (Mend), he was arrested in Angola five months ago on arms trafficking charges.
The group said that Mr Okah had been extradited on Thursday.
Mend claimed he was being held at a secret location in northern Nigeria, but ministers in Abuja did not comment.
The militant group came to prominence in the Delta two years ago when it cut Nigerian oil production by a quarter.
'Calm for now'
Factions loyal to Mr Okah have carried out a string of attacks in the region since his arrest last September.
Mend said in a statement: "For now, we appeal for calm to the peace-loving people of the Niger Delta as we all watch to see the government's next line of action."
The militants claimed Mr Okah's arrest in Angola was orchestrated by the Nigerian government, in part because he was sceptical of the government's talk of peace.
Mend is made up of a collection of armed groups, which are occasionally political, often criminal in their activities.
While other groups have called a ceasefire after being offered government incentives, Mr Okah's faction has remained active.
The BBC's Alex Last in Lagos says Mr Okah's detention has upset many of his old colleagues, and some have threatened to call off their self-declared truce.
What the government does with him now could have a real impact on the prospects of peace in Nigeria's oil-producing region, our correspondent says.