Shell has failed to fulfill its legal obligations to clear up oil spills that it has caused in Nigeria's oil-rich Niger Delta region, Amnesty International said on Nov. 3.

Oil pollution caused by corroded pipelines and crude theft has longed plagued the southwestern Delta, an impoverished region despite being home to much of Nigeria's oil and gas wealth.

Amnesty said the findings of a 38 page report were based on research conducted in the Boobanabe, Bomu Manifold, Barabeedom swamp and Okuluebu areas of Niger Delta's Ogoniland region, between July and September this year.

Spills in those areas date back several years. Researchers said they found waterlogged areas with an oily sheen, "patches of oil-blackened soil at several locations" and, in some cases, pollution "spreading into neighbouring land and waterways".

The human rights organisation called on Shell to change its approach to the way in which it cleans up after oil spills and urged the government to publish detailed information relating to such operations.

A spokesman for Shell's Nigerian unit, Shell Petroleum Development Company (SPDC), said it was difficult to respond to the allegations without having seen the report.

"SPDC JV is committed to cleaning up all spills from its facilities, irrespective of cause. This is equally the case in Ogoniland, despite the fact that we ceased producing oil and gas there in 1993," said spokesman Bamidele Odugbesan.

In recent years, Shell has tried to move away from onshore oil projects in Nigeria, which are plagued by industrial scale theft, security problems and pipeline spills, which have become a growing legal liability.

In September, Shell said its future investments in Nigeria would focus on natural gas for domestic consumption and export.

Oil pollution and a lack of development had fuelled in the past an insurgency by local groups targeting oil facilities that was ended in 2009 with a multi-billion state amnesty.

President Muhammadu Buhari has left it open in what form he might continue the amnesty beyond a December deadline.