Many of the rebel groups have now laid down their weapons
Two car bombs have been set off in the Nigerian oil city of Warri, where officials were in talks over an amnesty for militants in the area.
Witnesses said the explosions shattered windows at the state governor's office and sent officials fleeing for cover.
The militant group Movement for the Emancipation of the Niger Delta (Mend) had issued a bomb threat earlier.
Several armed groups recently agreed to an amnesty, but some Mend leaders rejected the government's offer.
A Mend leader announced in January that a temporary ceasefire was over.
In Warri, witnesses reported seeing huge plumes of smoke rising into the air.
There were no reports of injuries.
"There were car bombs - two of them," Delta State government spokesman Linus Chima told AFP news agency.
"The first one exploded just as the governors came in and the second one was about 30 minutes later."
"I think the intention is obvious, just to scuttle the talks and make it seem as if Warri in Delta State is not safe."
The BBC's Caroline Duffield in Lagos says the use of car bombs is a significant change in tactics from Mend.
She says it is not clear how many of the group's top commanders are behind this attack.
A source close to Government Tompolo, a leading Mend figure, told the BBC that he condemned the attack:
''This came as a surprise to us. We had no prior knowledge of this.
''We are calling on the security services to arrest those behind it.''
For years, armed groups have caused havoc in the oil-rich Niger Delta, abducting oil workers and sabotaging pipelines.
They claim they are fighting for a fairer share of oil wealth for local people.
But their critics say they use the money they get from illegal oil sales and ransoms to buy weapons and fund more militant activities.