Nigeria's main oil militant group has accused the army of launching air attacks on its allies' camps.
The Movement for the Emancipation of the Niger Delta (Mend) declared a unilateral ceasefire three days ago.
A Nigerian army spokesman told the BBC that he was unaware of any air strikes on Tuesday.
Mend said it would not be drawn by military tactics into jeopardising the peace process and would continue observing its ceasefire.
Groups such as Mend claim to be fighting for greater control over oil wealth in the impoverished Niger Delta, but they are accused of making money from criminal rackets and trade in stolen oil.
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Mend said camps belonging to the Niger Delta Patriotic Force and the Niger Delta Vigilante had been targeted in the attacks.
Earlier in the month, the militants had declared "war" on Nigeria's oil industry after a fierce military raid on one of their bases.
Mend declared a ceasefire after appeals from local leaders, but it warned it would end the truce if attacked by the army again.
"Mend will not play into the hands of the military by retaliating and putting the peace process in jeopardy at this time," the group said in an e-mailed statement, Reuters news agency reports.
Recent violence has been the worst in two years and on Saturday oil giant Shell was forced to declare a "force majeure" - which frees it from contractual obligations - on crude oil shipments from its Niger Delta facilities.
Nigeria's oil production has been cut by 20% because of unrest in the region over the past few years.
When President Umaru Yar'Adua came to power last year he promised to tackle the problems of the Niger Delta.
Recently, his government announced that a new ministry would be formed to deal with the crisis.
A Niger Delta minister is expected to be appointed this week when the president unveils his new cabinet.
(BBC)
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