Henry Okah describes his time in captivity: "I prepared myself for the worst"
The main rebel group in Nigeria's Niger Delta has declared a 60-day ceasefire in its attacks on the oil industry.
The Movement for the Emancipation of the Niger Delta (Mend) said it was responding to the freeing of rebel leader Henry Okah on Monday.
Mr Okah was released under an amnesty after spending more than a year in prison on charges including treason.
A government official welcomed the ceasefire, saying the government was ready for dialogue.
Speaking after his release, Mr Okah told the BBC that peace could be reached only through talks with the government.
Mend has been fighting to drive the Nigerian military out of certain areas of the Niger Delta and allow displaced people to return to their homes.
But its campaign has been tainted by violent kidnappings of civilians and the theft of oil.
The Mend ceasefire was set to come into effect from Wednesday. But it had only just been declared when the group warned it was in jeopardy.
According to a statement emailed to news agencies, Mend said it had information that the government was sending gun boats towards one of its camps.
'Ready for dialogue'
Asked whether the ceasefire covered kidnappings, Mend said it was intended to benefit oil workers - some of the main victims of kidnappings - and the oil industry.
Attacks by the rebels have seriously disrupted Nigeria's oil industry
It told the BBC that there must be no repairs to oil pipelines and buildings it had already blown up and warned the ceasefire would not cover them from being destroyed again.
Mend added that it would monitor its own ceasefire but would not enforce it for other militant groups.
There are hundreds of criminal gangs in the Niger Delta and it is unlikely that all of them will observe Mend's ceasefire, says the BBC's Caroline Duffield in Abuja.
Timiebi Koripamo-Agary, spokeswoman for President Umaru Yar'Adua's amnesty commission, said she welcomed the truce.
"The ceasefire was as a result of our contact with them," she told AFP news agency.
"We have made it clear to them that the president is honest and can be trusted and that the government is ready to dialogue with anybody on the need for peace and development."
The later Mend statement said it was alarmed at reports of a military force approaching one of its camps.
"Barely 12 hours into our ceasefire, the military Joint Task Force [JTF] has dispatched seven gun boats with heavily armed troops from Warri and are headed towards one of our camps located around the Delta/Ondo state border," the statement said.
"If this information from a very reliable source within the JTF happens to be true, the ceasefire will be called off with immediate effect."
On Sunday, just hours before Mr Okah was freed, militants in speedboats attacked the main oil depot serving Lagos - well away from the usual area of operations in the Niger Delta.
Mr Okah said he regarded the attack on the Atlas Cove jetty depot as a gesture, welcoming him to freedom.
He said that only a government offer of talks with the rebels would help bring about an end to the violent confrontation.
Numerous attacks by Mend on installations in the Niger Delta in recent years have seriously disrupted the Nigerian oil and gas industry.
The government offered militants an amnesty three weeks ago, promising a rehabilitation programme, including education and training opportunities.
The government's critics say the amnesty is unlikely to work because the unrest is not a straightforward political struggle.
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