"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.