Shell on Thursday said it had launched a review of its oil and gas assets in Nigeria's massively polluted Ogoniland region, resuming work in the area two decades after unrest forced the company to pull out.

The Anglo-Dutch oil major said the move was not part of an attempt to restart oil production in Ogoniland, describing it instead as a bid to comply with a 2011 UN report that called for one of the world's biggest ever environmental clean-ups.

"The intention is to determine the state of our facilities since we suspended operations in the area in 1993, and determine how best to decommission them," the head of Shell Petroleum Development Company of Nigeria (SPDC), Mutiu Sunmonu, said in a statement.

Spokesman Precious Okolobo told AFP the review was "a key step" in complying with the United Nations Environmental Programme report, which detailed the devastating impact that decades of oil pollution had brought to the southern region.

The report called for the oil industry and the Nigerian government to contribute $1 billion (762 million euros) to a clean-up fund for the region, adding that restoration could take up to 30 years.

Among the most condemned episodes in Ogoniland's past was the 1995 execution of renowned environmental activist Ken Saro Wiwa under the regime of dictator Sani Abacha. Wiwa had fiercely criticised Shell before his death.

His brother, Harry Wiwa, told AFP Thursday that Shell could be welcomed back to the region after the two-decade absence.

"If the purpose is to clean the spills, they are welcome but UNEP should supervise the exercise... The problem we have with Shell is that it is not socially responsible," said Wiwa, an activist with the Movement for the Survival of the Ogoni People.

Okolobo said the company has sent community outreach staff to the region in recent years, but the review exercise marks the first time oil and gas workers have been to Ogoniland since the pull-out.

The firm still controls at least seven oil fields in the area, as well as flowstations, gas plants and pipelines, according to the SPDC statement.

Nigeria's is Africa's top oil producer, but the country remains deeply impoverished, partly due to massive corruption in the energy sector.