Yvonne Ndege, Al Jazeera's correspondent in Lagos, said: "There are two news agencies reporting that Mend have declared a unilateral ceasefire and there are also two theories emerging as to what could have prompted this.
"One is that they are regrouping and rearming. The second is that their intelligence sources are telling them that the Nigerian army is preparing for a major assault on their bases.
"As of yet, there has been no official motivation, given by Mend, as to why they have called for this unilateral declaration of a ceasefire and there is still no reaction from the Nigerian government to this news," she said.
Shell, which has confirmed only two of the six attacks claimed by Mend, on Friday declared force majeure on exports from the Bonny terminal.
The legal move freed the company from delivery obligations in the wake of attacks on the oil terminal.
Mend's statement comes a day after the group said it had destroyed a Shell-run pipeline.
The group last week declared it was fighting a "war" against the oil industry after what it called an attack on its positions by the Nigerian army.
Nigeria is Africa's most populous nation, with 140 million people, and about 90 per cent of the country's foreign currency earnings come from oil and gas.
Amid the unrest in the oil-producing region's output has dropped to between 1.8 million and two million barrels per day, down from an average of 2.6 million bpd two years ago.
Mend said on Sunday it will renew attacks if the Nigerian military targets it again.
"We hope that the military has learnt a bitter lesson... The next unprovoked attack will start another oil war," its statement said.
Mend has said that more of Nigeria's oil wealth should be channelled back to the local population.