Dangerous heavy metals used in oil production in war-torn South Sudan have leaked into drinking water sources used by 180,000 people with life-threatening health risks, a rights group said Friday.

Toxicological tests carried out on hair samples from 96 volunteers living around the Thar Jath oil processing plant in South Sudan's northern Unity region revealed they were "highly intoxicated with pollutants such as lead and barium," said Klaus Stieglitz, from the German-based Sign of Hope organisation.

"There is a direct link between the contamination of the people and the activities of the petroleum industry working in this area," he said, adding that the research built on six years of hydrological tests by the group in the region.

"The total toxic stress -- as found in the hair samples -- of the human population of the area is life-threatening," said Klaus-Dietrich Runow, from Germany's Institute for Functional Medicine and Environmental Health, one of two separate independent toxicologists who assessed the samples.

Previous tests have shown "direct links" between oil drilling and drinking water contamination.

Despite the small number of samples analysed, the "homogeneity" of the results suggests they apply to "large sections" of the surrounding population, estimated at 180,000 people.

"There is a concern for health effects in the localities of Koch, Leer, and Nyal," Professor Fritz Pragst, from Berlin's Charite hospital said.

The areas, which have seen some of the heaviest fighting in more than two-years of civil war, changing hands several times, lie in the watershed of the White Nile river and the swamplands of the Sudd, one of the world's largest wetlands.

The UN has warned that the areas are at dire risk of famine.

- War, humanitarian crisis -

South Sudan is estimated to produce around 150,000 barrels a day -- down from 350,000 at independence in 2011 -- after many oil fields ceased operating since war broke out in December 2013 between President Salva Kiir and rebel leader Riek Machar.

Oil wells and facilities have been badly damaged in the fighting. The Thar Jath oil treatment facility, which handled oil from surrounding wells operated by the consortium SPOC (Sudd Petroleum Operating Company), led by the Malaysian giant Petronas, was abandoned days after war broke out.

Tens of thousands have died in the war, more than 2.3 million people have been driven from their homes and 3.9 million South Sudanese face severe food shortages.

The United Nations this week warned that the country's humanitarian crisis is worsening, with the warring sides "dragging their feet" in implementing an August peace deal.

South Sudan is currently negotiating a new oil pipelines deal with north Sudan, after slumps in global prices made Juba effectively pay to export crude.

The countries, which split in July 2011 after decades of war, agreed then to a fixed fee for the use of export pipelines.

But with global prices so low, the fee was more than it earned for the oil itself, meaning South Sudan lost money on every barrel it sold.