Several state governors and other government officials had gathered at the Delta state governor's offices to discuss the implementation of an amnesty programme for fighters launching attacks against oil workers and oil installations in the delta.
"Two bombs exploded on the Government House expressway, one about 400 metres from Government House where the meeting was taking place, and another about 100 metres from the building," Tunde Forsythe, a government official who was just outside at the time of the explosions, told the Reuters news agency.
Al Jazeera's Yvonne Ndege, reporting from Abuja, the Nigerian capital, said the building housing the offices of Emmanuel Uduaghan, the governor, had been evacuated.
"The entire area is pretty much on lockdown as fear and concerns grow about the fact that one of three planted bombs has not yet gone off, she said.
"We also know that some of the oil companies operating in Nigeria are tightening security. Fear in the area is palpable.
"People are concerned that this event is a sign that Mend is planning to return to acts of violence to get their message across."
In its statement, Mend said the bombs were part of a new wave of attacks that would sweep across the Niger delta.
"The deceit of endless dialogue and conferences will no longer be tolerated,'' the group's statement read.
Violence had subsided in the Niger delta since Umaru Yar'Adua, the president, last year offered the fighters an unconditional amnesty if they laid down their arms.
But the amnesty started to stall after Yar'Adua left for three months of medical treatment in Saudi Arabia last November.
Our corespondent said the purpose of Monday's meeting was to discuss the post-amnesty programmes Yar'Adua came up with before falling sick.
"The feeling is that those post-amnesty programmes in which militants were supposed to be re-educated and trained and integrated back into civilian life are pretty much dead in the water," she said.
"This attack by Mend really is a signal to Nigeria's leaders of their frustration."
Yar'Adua returned to Nigeria last month but remains too sick to govern and the programme has made little concrete progress.
Fighters in the Niger delta have launched attacks and fought government troops since January 2006.
They demand that the federal government send more oil-industry funds to
Nigeria's southern region.