A Nigerian environmental group on Monday claimed an oil spill from a pipeline operated by Italian firm ENI had badly polluted an area in the south of Africa's largest oil producer.

The spill which reportedly occurred on September 27 is said to have polluted the swamps of the Ikeinghenbiri area of Bayelsa state in the main oil-producing Niger Delta region.

"The volume of the spill is very high and in some cases it is difficult to separate the crude from the water," Environmental Rights Action field monitor Morris Alagoa told AFP a day after he visited the village.

The group's executive director, who is also chairman of Friends of the Earth International, Nnimmo Bassey, said, "I understand it's a very severe spill."

Alagoa said he found that "in some places the whole length of the swamp is black (with oil)."

He could not give an estimate of the size of the area polluted or the volume of crude spilled.

The cause of the spill could not be immediately established. ENI did not respond immediately to an email asking for comment while an official from the government agency responsible for oil spill detection refused comment.

ENI said earlier this month that one of its pipelines had been sabotaged and was undergoing repairs, though it was unclear if that incident was the cause of the pollution reported on Monday.

Pipeline damage and resulting spills are common in the Niger Delta region, often as a result of oil theft to feed a lucrative black market.

A UN report in August said decades of oil pollution in Ogoniland, another area of the Niger Delta, may require the world's largest ever cleanup.