Shell on Friday deployed ships with dispersants and planes in a bid to mop up one of Nigeria's worst offshore oil spills in recent years, a spokesman said, amid fears it could soon reach the shoreline.

A Shell spokesman in Nigeria said five vessels and two aircraft had been deployed to attack the oil slick, with the company estimating the amount of the spill at its Bonga field at less than 40,000 barrels.

The leak has been stopped since the company became aware of it on Tuesday.

"Investigation is going on," Tony Okonedo said. Asked whether ships had been deployed as planned on Friday, he said, "Five vessels, two aircraft ... oil spill response specialists and other personnel are involved."

The company says the slick has been thinning and breaking up, but the information was difficult to verify independently.

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

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 Wednesday morning it published on its website, estimated the slick was 70 kilometres long and 17 kilometres wide at its widest.

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