The main militants group in Nigeria's oil producing Niger Delta say it will not extend its month-long ceasefire which expires on Tuesday.
The militants want locals to benefit from the oil wealth
The Movement for the Emancipation of the Niger Delta (Mend) says it has been kept on the sidelines of government-led talks about the region's future.
Mend also condemned the killing of civilians during a recent military crackdown in the area.
Mend called the truce to give the new Nigerian leader time to set up talks.
The BBC's Abdullahi Kaura in Nigeria's oil capital Port Harcourt says the militants are likely to resume their attacks on oil installations and kidnapping of foreign workers.
But he says a second Mend faction based in Warri appears to want to maintain the ceasefire.
Other criminal armed groups in the region have continued sporadic attacks and kidnappings in the region during Mend's truce.
The unrest has led to a 25% cut in oil output from Nigeria - Africa's largest producer.
"We cannot bear to pretend all is well," Mend spokesman Jomo Gbomo said in an e-mail to the BBC, pointing out that talks with the new government of President Umaru Yar'Adua had achieved little so far.
Last month, Nigerian troops attacked an oil facility held by militants in Ogboinbiri village in Bayelsa State killing 12 militants and two civilians, military sources said.
Several oil workers held hostage were rescued in the operation.
"It is clear that the families of these victims won't get justice," Mend says.
The militants say Niger Delta residents see little benefit from the oil which is extracted from their region.