Nigeria's main rebel group has claimed it launched a series of attacks on the Nigerian army, killing 29 soldiers, but the military has denied there has been any fighting.

The Movement for the Emancipation of the Niger Delta (Mend) said in an email statement on Saturday that near-simultaneous battles had taken place in the three main oil producing states of southern Nigeria.

The group reported six of its own fighters were also killed in the clashes, which Mend says it launched as reprisals for attacks it alleges the military carried out on civilians.

But Lieutenant-Colonel Rabe Abubakar, a military spokesman for Bayelsa and Delta - two of the three main states in the Niger Delta, - denied the army was under attack.

"All of our men are intact ... We are not under attack. We are not engaged in any fighting in Bayelsa or Delta states," he said.

Mend members said the attacks in Rivers, Bayelsa and Delta states came shortly after sunset, and involved fighters wielding machine guns, rocket propelled grades and anti-tank missiles.

The Niger Delta region is a vast network of narrow creeks and remote villages, and initial reports of fighting are often confused.

It was not immediately possible to independently verify the claims made by either side.

The Niger Delta is the heartland of Nigeria's oil industry, which is currently pumping around 1.9 million barrels per day, making it the world's eighth biggest oil exporter.

Insecurity in the region has cut the West African country's output by around a fifth since early 2006, when Mend began blowing up oil pipelines and kidnapping foreign workers, helping push up world oil prices.