21 July 2016 Last updated at 20:33 BST
The targeting of oil facilities has reduced the country's output and threatened its fragile economy.
So what is behind the renewed violence? The BBC's Martin Patience has spoken to one of the militants.

Concerns include physical, psychological and financial abuse.
Scottish Borders Council's social work director Andrew Lowe admitted cases dealt with by the authority's adult protection unit had "rocketed".
However, he said changes to reporting methods and greater awareness of the issue had contributed to the increase.
The council report shows a sharp rise in significant incidents and concerns over the care of the elderly and those with difficulties.
Mr Lowe said he anticipated they would keep rising - but pointed out a different method of reporting which sees all medical issues reported as adult protection concerns.
He said he believed the systems now in place were more robust and that there was a greater awareness of protection issues for vulnerable adults.
"One of the heartening things to see is the source of referrals - we are getting many more now from independent agencies than we ever used to," he said.
"It used to be that the referrals came from either social workers or family or whatever.
"We now see a range of all the different agencies - health board, GP, private sector, self-referral - they are all putting referrals in and that, to me, is healthy.
"That's a sign of public awareness of this as an issue."
He added that there was always more that could be done.
"All we can hope to do is have a good and effective system, good policies, and to make sure that we are raising the profile and ensuring people are aware of the vulnerability of people in our community," he said.
He urged the public to contact the authorities if they saw something which caused concern.
"It doesn't matter if you are wrong," he said.
"If your hunch is wrong we can make a discrete inquiry and nothing will happen and there will be no threat to you.
"But if you see something and you think it is wrong, I think acting on that is very helpful."
Oil militancy has made a comeback in Nigeria with a recent wave of attacks carried out by the Niger Delta Avengers.
[NEXT_CONCEPT]
The number of reports of vulnerable adults being harmed in the Scottish Borders has risen sharply, according to a social work report.