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