An environmental group said Tuesday that an oil slick had approached Nigeria's coastline after a major Shell spill last week, but the company insisted that its spill had been largely dispersed.

Nigerian group Environmental Rights Action, which closely monitors oil spills in the country, said oil was reported along the shoreline of fishing communities in Bayelsa state as well as Delta state.

The group said it sent monitors out after reports from fishermen. It said it suspected the oil had come from the Shell spill, but the claim could not be independently verified.

"In the course of the visit, spreading slick was sighted close to the coastline of Odioama and along St. Nicholas," it said in a report that included photos of streaks of what appeared to be oil just off the coastline.

"The footprint comes from the ocean," the group's head Nnimmo Bassey told AFP. "We suspect it is from Bonga."

Shell has said the spill from its offshore Bonga field, which it became aware of on December 20, amounted to less than 40,000 barrels and that it had been "largely dispersed."

Five vessels and two aircraft had been deployed to spread chemical dispersants.

A Shell spokesman in Nigeria told AFP that "if there was any more spill found on the coastline, it must have come from a third party."

"We found a third-party spill and we have told our team on the ground to clean it. If there is still a spill on the shoreline, it is a third-party spill," Precious Okolobo said.

Bonga, which has a capacity of 200,000 barrels per day, is located some 120 kilometres off Nigeria, Africa's largest oil producer and an OPEC member. Production has halted at the field.

The company said the source of the leak was a flexible line linking a production vessel to a tanker.

It was Nigeria's worst offshore spill since a 1998 Mobil incident, officials said, though onshore leaks have been estimated at levels far worse since that time in the oil-producing Niger Delta.

Environmental group SkyTruth, using satellite imagery from December 21 it published on its website, estimated the slick had been 70 kilometres long and 17 kilometres wide at its widest.

It said it had covered 923 square kilometres (356 square miles) of ocean.