Mend had mostly observed a voluntary ceasefire since May to allow for talks with the government of Umaru Yar Adua, Nigeria's president.
Since last year, dozens of people have been killed, thousands of foreign workers have left and oil output from Nigeria, the world's eighth largest exporter, has been cut by a fifth, raising world prices.
Decades of neglect
Mend says it is fighting against decades of neglect and marginalisation of Nigeria's oil heartland.

"We will not sit back and allow our birthright to be exchanged for a bowl of porridge"

"Jomo Gbomo", Mend leader

The Niger Delta is home to all of Nigeria's oil, responsible for 95 per cent of its hard currency earnings, but most people there live in poverty.
Mend's email accused the Nigerian government of trying to divide and rule the inhabitants of the delta, and attempting to bribe fighters and leaders from the region.
"We will not sit back and allow our birthright to be exchanged for a bowl of porridge," the email said.
The email was signed "Jomo Gbomo" and sent from an address used by Mend for the past two years to communicate with the media.
Security sources had said that Gbomo was Henry Okah, a factional leader arrested in Angola on September 3 on arms trafficking charges, but the email denied they were the same person.
The Reuters news agency reported Okah's wife as saying she suspected the Nigerian government was behind her husband's detention in Angola.