The country's oil production has been severely disrupted by the attacks.
US oil giant Chevron shut down an offshore platform this month after an attack claimed by the Avengers group.
Many militants joined an amnesty programme in 2009 after an insurgency in the oil-rich delta region.
Nigeria has long been Africa's largest oil producer, but its economy is currently facing difficulties due to the recent drop in global oil prices and its output is now behind that of Angola.
Most of Nigeria's oil wealth comes from the Niger Delta, an area which remains poor and underdeveloped.
Previous insurgent groups said they were fighting so local people could benefit more from their region's natural resources.
Oil spills have also resulted in environmental devastation over the years.
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