Shell was on Tuesday accused of making false claims about the extent of its oil spill clean-up operations in Nigeria and urged to take more action to help worst-hit communities.

Amnesty International and the Centre for Environment, Human Rights and Development (CEHRD) charged the oil major with failing to implement recommendations from a critical 2011 UN report.

The claims came a week before the 20th anniversary of the execution of Ken Saro-Wiwa, who helped bring the extent of oil-related ecological damage in Nigeria to world attention.

Wiwa and eight community leaders from Nigeria's southern Ogoniland area were hanged on November 10, 1995 after being convicted at a secret trial of murdering four local chiefs.

Nigeria's then military government ignored pleas for clemency from world leaders.

Special vigils and protests outside Shell petrol stations were planned in the run-up to next week's anniversary, Amnesty said.

But campaigners said communities in the creeks and marshes of Nigeria's oil-producing southern delta region were facing the same problems Wiwa highlighted two decades ago.

"Twenty years; nothing has been done," Fyneface Dumnamene Fyneface, a human rights and environmental activist from Port Harcourt, said in a statement.

"Ogoniland is still polluted... no clean-up has been done... justice has not been achieved. Twenty years and what they fought for has not been addressed. That cannot continue."

- 'False claims' -

Amnesty and the CEHRD's claims come in a new report, "Clean It Up: Shell's False Claims about Oil Spill Response in the Niger Delta".

The 38-page document said most of the recommendations of a UN Environment Programme (UNEP) report had not been implemented since its publication five years ago.

Thirteen out of 15 areas visited between July and September this year were still "visibly polluted" or contaminated, despite claims to the contrary by Shell and the government.

The inadequate clean up left thousands of people "exposed to contaminated land, water and air, in some cases for years or even decades," said Amnesty researcher Mark Dummett.

Either no clean-up had been carried out or had been done badly, the groups suggested, adding that other spills may have happened since.

Activists have long highlighted the problem of oil spills in Nigeria and the impact on local populations, most of whom rely on fishing and farming to earn a living.

Ogoniland has become a symbol of Nigeria's troubled relationship with oil, which has made government officials and oil majors rich but done little for local people.

Amnesty has previously accused Shell of not implementing the UNEP recommendations, which said at the time the clean up could take 30 years and cost $6 billion.

The latest report also criticised the government watchdog the National Oil Spill Detection and Response Agency, accusing it of certifying areas as clean when they were not.

The agency should be better-resourced and given more powers, as well as forced to become more accountable and transparent about its operations, the authors argued.

- Shell 'committed' -

Shell has not pumped oil from Ogoniland since 1993 when it pulled out because of unrest but still runs pipelines through the area.

Activists say the pipelines are poorly maintained and old, making spills more frequent.

Amnesty and the CEHRD urged Shell to conduct proper clean-up operations but the company's Nigerian subsidiary itself said it rejected the groups' claims.

In a letter published in the report the Shell Petroleum Development Company of Nigeria said it was "committed to the implementation of the UNEP report".

It had "initiated action on all the recommendations addressed directly to it", it added.

"We disagree with the assertions made with regard to implementation... and would like to reiterate that we have consistently and publicly reported our actions in this regard as well as highlighted ongoing challenges of crude oil theft and illegal refining," the letter read.