A report by the organisation, Human Rights Watch, says that although the ultimate responsibility for abuses in the southern region lies with the Nigerian government, oil companies also have a duty to try to stop them.

The organisation argues that the oil companies, which operate joint ventures with the Nigerian government, can influence policy in the delta, but have chosen not to do so.

Eyewitness accounts

The Human Rights Watch report contains harrowing eyewitness accounts of extra-judicial killings, torture and rape, which it alleges are still being carried out by the security forces in the delta.

The Niger delta has seen riots and refugee crises in recent months

And the authors of the report allege that the human rights situation in the area has actually got worse since the country's former military dictator, General Sani Abacha, died last June.

His successor, General Abdulsalami Abubakar, has promised to restore democracy to Nigeria.

On Saturday, Nigerians will be voting in presidential elections after more than 15 years of military rule.

Difficult environment

Oil companies, including Royal Dutch Shell, Chevron and Texaco, say they are committed to improving the welfare of the people there.

General Obasanjo is the current favourite in the presidential race

They argue that they pay large sums of money through taxation to the Nigerian government, which then has a responsibility to use that money wisely.

And the report acknowledges the companies are working in a difficult environment, where successive military governments have paid little attention to the people of the Niger delta.

Local communities in the Niger delta have been campaigning against the Nigerian government and foreign oil companies.